We discuss the distribution of weak vowels on the basis of the position in the word.
Word-finally, three vowels occur in GA: /ə, i, oʊ/.
Internally, before vowels:
Internally, before consonants:
Word-initial syllables occurring before a strong syllable are a special case. All the weak vowels may occur here, but they are subject to variable reduction. That is, we get weak vowels that are ‘on their way’ to /ə/, which is the most reduced (‘weakest’) vowel of the language. Take the word notation, for example. In formal speech it will be /noʊˈteɪʃən/, but in less formal speech /nəˈteɪʃən/, or perhaps something in between these two. Similarly, we may get /suˈpɪriər/ for superior when pronounced carefully, but /səˈpɪriər/ when pronounced more casually, or again, with some vowel quality in between. Or emit, which may be /iˈmɪt/ or /əˈmɪt/. Such variable reduction may also occur with strong vowels. A careful pronunciation of abyss will be /æˈbɪs/ (more commonly /əˈbɪs/). Thus, these open word-initial syllables are weak, but the vowels tend to be subject to some variation. An exception is raccoon /ræˈkuːn/, which cannot be */rəˈkuːn/.
When the word-initial syllable is closed, it is strong, and no weak vowels are tolerated. Thus, ambivalent /æmˈbɪvələnt/ will never be */əmˈbɪvələnt/. Other examples are vulgarity, technique, pragmatics, September, October, pontoon. (Oh, yes, there is an exception, of course: Kentucky, pronounced /kənˈtʌki/.)
‘Latinate’ prefixes are always weak, independently of whether they constitute open or closed syllables. So retain will have /i/ or /ə/, not the strong vowel /iː/, in its first syllable. Other such prefixes are ab-, ad-, ex- (but not the ex- meaning ‘former’) be-, con-, ob- (not re- meaning ‘again’), sub.
Some speakers may make a distinction between /ɪ/ and /ə/ in the same context, and making a difference between act of God and active God; except and accept; effect and affect; or parrot and pair it; Lenin and Lennon. You need not make such distinctions.
The ‘word’ that is referred to by terms like ‘word-final’ and ‘word-initial’ is strictly speaking the phonological word. For example, there are two phonological words in tablecloth (or if you prefer table cloth, or perhaps table-cloth). That is, each of the words that a compound word is composed of is a phonological word, regardless of whether you use a hyphen or space or nothing between the component elements. Prefixes with independent meaning, like ex– ‘former’, neo– ‘new’, mini– ‘small’ also form independent phonological words, as in ex-priest, neo-Marxist, mini-city etc. Thus, final weak /i/ occurs at the end of mini just as it occurs at the end of city. The phonological word corresponds to an element that (a) can be an independent word table, cloth, city, or (b) is a prefix with independent meaning, like mini, neo, ex, etc. In addition, suffixes like –hood, –ness, –wise are not included in the preceding phonological word, so that /i/ will occur in nanny-hood, happiness, money-wise.