Reduction: strong and weak forms

Reductionis a process of weakening, shortening, or disappearance of vowel sounds in the unstressed positions. Reduction is closely related to such notions as "word stress" and "sentence stress". English speech moves in regular rhythmic beats from stress to stress. To maintain the rhythm of speech, words that carry information, are marked by the stress, whereas words that show grammatical relationships are not marked by the stress.

So, in English there exists an evident contrast between stressed and unstressed syllables:

a) words that bear (carry) the main information are generally stressed and have
mostly strong forms, they are called "notional words",
and they comprise the following parts of speech: nouns, adjectives, notional
verbs, adverbs, numerals, interrogative pronouns, demonstrative pronouns;

b) words that link the content words and thus help to form an utterance are
called "form words" or "structural words", and they include such parts of
speech as: articles, prepositions, conjunctions, particles, auxiliary verbs, modal
verbs, personal pronouns, possessive pronouns, reflexive pronouns, relative
pronouns and the indefinite pronoun "some" (denoting indefinite quantity).

Form words have 2 forms of pronunciation:

1) strong, or full forms: 2) weak, or reduced forms:

a) when they are stressed: when unstressed, each functional
Ex.: He will do it (and not anybody else), [hi: wIl du: It] word usually has more than one weak form:

b) in careful speech, even though Ex.: he [hi'], [hi]
half-stressed or unstressed: Ex.: He will do it. [ hi'l du: It],

Ex.: He will do it. [hi: wIl du: It] or [ hIl du: It ].

In short, reduction is the process where sounds are not clearly articulated or even disappear in certain contexts:

strong forms(stressed) reduction weak forms (unstressed)