There are two kinds of stress in English: word stress and sentence stress. Word stress means that some syllables in polysyllabic words are stressed more than others; sentence stress means some words in a sentence are stressed more than others.
It is sufficient to distinguish two degrees of word stress: primary and secondary. Unstressed syllables tend to be reduced to [ə], [ɪ] or [ʊ].
Most two-syllable words are stressed on the first syllable (never, breakfast, Monday), unless they have a prefix, in which case they are usually stressed on the second syllable (display, exceed, device, explain, believe, intent).
Compound nouns are usually stressed on the first element (bird’s-nest, drugstore, thoroughfare, weatherman); compound verbs are usually stressed on the second element (understand, overlook, outrun).
Many words are identical except that they are nouns if stressed on the first syllable, but verbs if stressed on the second (concert, increase, overflow, object, contrast, conduct, conflict, context, contract, convert, desert, incline, insert, insult, object, permit, present, progress, project, protest, rebel, record, survey, suspect). If the word ends in –ate, it will be unstressed and pronounced [ət] or [ɪt] if it is a noun, and stressed and pronounced [et] if it is a verb (advocate, aggregate, alternate, animate, appropriate, approximate, deliberate, desolate, elaborate, estimate, graduate, intimate, moderate, precipitate, separate).
Words ending in –tion, -sion, -ic, -ical, -ity, and –graphy are stressed on the immediately preceding syllable, which can cause a shift of accent (contribute, contribution; biology, biological; public, publicity; photograph, photographer)
-self is stressed (myself, yourself, himself).
-teen has secondary stress when counting but primary stress otherwise (fourteen, fifteen).
Within the larger unit of the sentence, it is normally the content words that are stressed, and the function words that are not. Content words include nouns, most verbs, adjectives, adverbs (including not and verbs contracted with not), demonstratives (this, that, etc.), and interrogatives (who, when, etc.). Function words include articles (a, the), prepositions (to, of, etc.), personal pronouns (I, me, he, etc.), possessive adjectives (my, his, etc.), relative pronouns (who, that, etc.), conjunctions (and, but, etc.), one as a pronoun (the red one), and the verbs be, have, do, will, would, shall, should can, could, may, might, and must (except when they end a sentence, as in a tag question).
Compound nouns written as two or more words have sentence stress on the first word to distinguish them from sequences of adjective + noun, e.g., French teacher, social worker, metal cutter), except when the second word indicates what the first word is made of, in which case both words are stressed (gold watch, apple pie).
Phrasal verbs are stressed on the second element (put on, look up, write down).
One-syllable words that do not receive sentence stress, like unstressed syllables, tend to be reduced to [ə], [ɪ] or [ʊ]. Since this includes function words, which are the most frequent words in the language, it means that much of the vowel quality of unstressed words and syllables is lost (or neutralized) in the stream of speech. The faster one talks, and the more informally, the more reduction there will be.