Exceptional/exceptionable

Exceptional is, of course, something that is an exception, unusual, abnormal:
We had an exceptionally wet summer three years ago.
Exceptionable is mainly used in the negative form unexceptionable. It means acceptable, not open to objection: I have read the conditions. They seem unexceptionable.
To take exception to something means to disapprove, raise an objection. You will probably find exceptionally good weather unexceptionable.

Fast/quick

These two are not completely interchangeable. Fast refers to speed of movement or action:
A fast train. A fast run.
Quickrelates to the length of time an event or action takes:
A quick meal. A quick reply.
Using both adjectives in one sentence, you could say: They had a quick crossing because the boat was fast. He had a quick meal because he is a fast eater.
Exception department: quick can mean speed if the movement or action is not sustained, abrupt: Try to get that window seat! Quick! or He is an odd kind of man, with very quick, nervous movements.

Farther/further

Good news! Further can now safely be used in place of fartheralthough there is a difference according to the dictionary. Farther is the comparative of far and thus relates to physical distance:
London-Eastbourne is farther than London-Brighton.
Nobody will be worried if you use further here. Further means additional, other, subsequent:
We have to await further developments.
Floor/storey (Am.: story)
Floor indicates the position in a building: second floor, top floor, etc. Storeyrefers to the height of a building: a ten-storey apartment block.

Historic/historical

Historic refers to something important that is or will be remembered in history, recorded by history: a historic meeting, historic decision, historic voyage, historic landmark, etc. Historical is the adjective for all other purposes when you mean to do with history: a historical play, historical novel, historical costumes, etc.

Human/humane

Humanis the more frequent adjective when referring to matters concerning homo sapiens: human habitation, human failings, human ancestry, etc. Humane means benevolent, compassionate: humane treatment is decent treatment.

In future/in the future

In future means from now on, starting now: All goods will in future be sent by our own transport.
In the future is more vague and means at some unspecified later date:
We hope to be able to send all goods by our own transport in the future. (As soon as we can afford three trucks.)
While talking about 'the future', here is one small point you may find interesting. If today is the 5th of the month and you want to say that something will happen on the 26th, you have three ways of putting it:
in three weeks; in three weeks from now; in three weeks' time.
When using the last version, please don't forget the apostrophe in writing: In five hours'/days'/months'/years' time.
if/whether
In many constructions these two words are indeed interchangeable:
I am not sure if this is possible.
I am not sure whether this is possible.
The main difference is that whether is always assumed to be followed by or not. This means that in questions or requests an answer is usually expected. Take these two almost identical sentences: Let me know if you can come.
The stress is here normally on the word know. The speaker or writer tells you here: If you can come, please let me know. (If you can't, don't bother to notify me). On the other hand, someone may tell you: Let me know whether you can come. This says: Please let me know whether you can come or not. An answer is required.

Imply/infer

Many people think that these two mean the same thing. There is a difference worth remembering.
Imply is what the speaker or writer does: says, suggests, insinuates, hints at something that can be interpreted in a certain way. Inferis what the listener or reader does: concludes, guesses, deduces, thinks. Taking the two together, you could say:
When she implied that she was very busy, he inferred that he was not welcome.
The two nouns are implication and inference. (Inference has the stress on the first syllable. The remainder is pronounced like the same syllables in 'reference' )

Incredible/incredulous

Both have something to do with believe, as anyone will tell you who has ever battled with Latin.
Facts, events, reports, and other impersonal things are incredible (or credible): they are hard to believe. People can be incredulous, i.e. they don't believe what they read or hear. The two together: He was incredulous when he heard their incredible story.

Insulated/isolated

English has two words, where other languages only have one. Insulated is the technical word: protected against electricity, heat or cold. Isolated is the general adjective. It is used for anything that is separated, kept apart: When the tide rose, he found himself isolated on a tiny rock.

Last/latest

Last denotes final or most recent. This can be confusing: her last book can mean that she never wrote any others or that it is her most recent literary effort. Latestmakes the position quite clear; it means the most recent of several, leaving open the possibility of more to come: our latest model, their latest publication, her latest hairstyle.

Less/fewer

An interesting pair. Less is followed by a noun in the singular and is used when referring to extent, degree, and quantity in bulk. Fewer is followed by a noun in the plural and refers to quantity in terms of units. A few examples: less milk; fewer bottles; less help; fewer assistants; less money; fewer pesetas; less expense;
fewer bills; less weight; fewer kilos

Libel/slander

Both mean the same: making false, damaging statements; telling lies about a person or company.
Libel, however, is something written, usually a newspaper article. Slander is spreading all that nasty information verbally. As it is much easier to prove written defamation than word-of-mouth comment, you will sometimes hear about a libel suit; hardly ever about legal action involving slander.

Lie/lay

These two are often mixed up in English-speaking countries, mainly in the past tense.
There are two separate verbs: to liemeans to recline, to be in a horizontal position. It is irregular: to lie - lay - lain. He likes to lie in bed until lunchtime. The book lay on the shelf. The ring must have lain on the counter.
It is an intransitive verb, i.e. you cannot add a direct object. You may lie on the floor, if you like that sort of thing; but you cannot lie something on the floor. You then need the other verb: It is to lay, which means to place, to put. It is irregular: to lay - laid -laid. It is transitive, i.e. it can be followed by a direct object: Let me lay the table. She must have laid it there by mistake. The ostrich laid an enormous egg.
A frequent mistake is saying he lay it on the table. As you now know, it should be: He laid it on the table. Note: Just to complete the picture, there is also a third verb to lie, which means telling something that is not true. It is regular: to lie – lied - lied,

Long/lengthy

These two often mean the same, except when referring to something you have to read or listen to, when lengthy can imply that you were bored:
He produced a lengthy report after his trip to Korea. Many pages, but not terribly entertaining.

Much/many

In the same way as less and fewer, much applies to bulk, mass, an unspecified quantity. Manyare objects you can count:
much money, much demand, much traffic, much food.
many guilders, many orders, many cars, many dishes
The same definition applies to much worry/many worries and much trouble/many troubles. Much worry is the total extent of your problems. Many worries are the individual headaches that bother you.

Murder/assassinate

The difference is simple: you and I are murdered. Statesmen and other important people are assassinated.

Offer/quotation

An offer is more general. It can be verbal or in writing. It is often applied to bulk goods with a variable price: We are interested in an offer for 250 tonnes of prime bleached sulphite pulp. The American unit of weight is still called ton.
A quotation is more formal. It is always in writing and is the best term for a detailed proposal: Please let us have your quotation for Model KLB 2000 with power feed, rotary table, and grinding attachment. An offer is often based on a price calculation, a quotation on a fixed price list.
Outside the office you also have a verbal offer when you tell someone how much you are prepared to pay. For a used car, for example. An offer at an auction is called a bid.

Official/officious

No problem with the adjective official, which means the same as in other languages: properly authorized. Officious describes a person who is over-keen to give service, who rushes around and upsets people. The best noun would be busybody. An officious waiter can ruin your carefully planned business lunch, if your guest becomes irritated and is in no mood to discuss the proposed contract.

Older/elder

Olderis the general-purpose comparative form of old: old - older -oldest. Elder and eldest refer to family members and are always attributive adjectives: my elder brother, my eldest sister. You cannot say my brother is elder than I.

On time/in time

On time means punctual: The train is on time.
In time means not late, before the last minute. In good time gives you a little extra:
We should get a window seat if we get there in (good) time.

Passed/past

Identical pronunciation can cause mistakes; passed is a verb form, the simple past or past participle of to pass:
She passed the test. They passed over the bridge.
Past is here a preposition, applied to time or space:
It is now half past ten. The ball whistled past the goal post.

Practical/practicable

Practical has to do with reality, with practice, with a good idea: it seems the only practical solution.
You are a practical person if you can knock a nail into the wall without hitting your thumb. Practicable means possible, something that can be put into practice: This idea is unfortunately not practicable. This means it can't be done. A practical suggestion may not be practicable.
Negative versions: unpractical and impracticable. (Impractical can be found in American English.) Practically is colloquial for almost: The book is practically finished.

Principle/principal

As these two sound completely alike, spelling mistakes are not uncommon. Principle is a noun and means the motive guiding an action or attitude.
He looks narrow-minded, but we have to respect his principles. It can also be the basic element: I am not worried about the extra 50 cents, it's the principle that bothers me. Principal as adjective means main, chief, first, foremost: Their principal export is bananas. As a noun it means the head of an institution, usually of a school or college: Let's see the Principal about Willie's bad exam results.
In business an agent may refer to the firm or person he represents as his principals: We are authorized to sign on behalf of our principals.

Question/query

A question is a straight request for information. The person asking it may know something about the subject; he may know nothing. A query is the result of some doubt in the mind of the speaker or writer who usually knows something about the subject.
Clever you and I, listening to the lecture on The Advantages and Risks inherent in Pressurized Water System Nuclear Reactors', may afterwards raise a query. Jack over there, who slept solidly throughout the talk, will wake up and ask a question.

Readable/legible

Readable is easy to read because of style:
The highly complicated subject was treated in a readable manner. If you want to make sure that your books are read by a wide audience, you must make them readable, i.e. easy to understand, a pleasure to read. Legible is also easy to read, but because the printing or writing is clear:
Some pages in this old bible are hardly legible.

Small/little

Small is the neutral, general-purpose word, but little has an emotional element and conveys a personal attitude to the object.
They have a small fox terrier describes size in a detached, matter-of-fact way. You should see their little cocker spaniel indicates that the speaker is rather fond of the little fellow.
Another use of little, often unconnected with physical size, is as reinforcement of a negative opinion. When you say 'He is a nasty man!' you are obviously not over-impressed by his charm. When you hiss 'He is a nasty little man!', he must be very nasty indeed.

Large/big

The difference is the same as that between small and little. Largeis for sober comment; big for a more personal or emotional opinion.
An advertisement for a house, for example, may mention large dining room . When you have seen it, you are likely to tell your friends: 'it's got a big dining room'.

Stimulus/stimulant

As you can guess, these two words have something to do with getting things to move a bit faster, raising the interest level and other developments in the right direction. Stimulus is the abstract term:
His enthusiasm was a tremendous stimulus to all of them.
Stimulant is the concrete article, usually something you swallow to prevent you from going to sleep.

Tall/high

Both mean a certain distance from the ground. Tall implies a narrow base, something slender: a tall chimney, a tall mast, a tall person. High has a base of a certain width: a high wall, a high mountain, a high fence.
When you are in doubt about someone who is tall but by no means slender (Fatty weighs 150 kilos), call him big. If a person is described as 'high' it means that he or she has had too much alcohol or drugs.