I am wondering when the KIT vowel (or the FOOT vowel) is actually a
strong vowel and when it is a weak vowel. I think it is weak when occurring in an unstressed syllable […] Thus, it should be strong when occurring in syllables that receive either primary or secondary stress. My questions is: is it weak when occurring in a monosyllabic unstressed structure word like "in" within a sentence?
What about content words which […] do not receive stress for [some] reason?
The general rules about strong (= stressable) and weak vowels in English are that
1. in a stressed syllable you can only have a strong vowel;
2. in an unstressed syllable you can have any vowel.
It would be nice if vowels were always weak in unstressed syllables. But clearly that is not the case in English (unlike, say, Russian), as shown in famous pairs such as modest ˈmɒdɪst (last vowel weak) but gymnast ˈdʒɪmnæst (last vowel strong); informant ɪnˈfɔːmənt but torment (n.) ˈtɔːment; Thomas ˈtɒməs but commerce ˈkɒmɜːs, etc.
Most noticeably, the vowel in the less prominent part of a compound is strong despite being unstressed, e.g. bedsheet ˈbedʃiːt, tentpeg ˈtentpeɡ, kettledrum ˈketl̩drʌm. But notice also non-compounds such as colleague ˈkɒliːɡ, phoneme ˈfəʊniːm, hypotenuse haɪˈpɒtənjuːz.
Some analysts (particularly Americans) argue in the other direction, claiming that the presence of a strong vowel is sufficient evidence that the syllable in question is stressed. In the British tradition we regard them as unstressed.
(There are a few exceptional compounds in which the secondary element DOES weaken, notably some in -man and -land, as milkman ˈmɪlkmən, Finland ˈfɪnlənd. Compare, though, snowman ˈsnəʊmæn, Nagaland ˈnɑːɡəlænd with no weakening.)
The weakening process converts what would otherwise be a strong vowel into a weak vowel. This is most obviously seen in function words with distinct weak forms. (All examples from BrE.)
them strong ðem, weak ðəm
from strong frɒm, weak frəm
us strong ʌs, weak əs
are strong ɑː, weak ə
for strong fɔː, weak fə
When ordinary lexical words bear no sentence stress, there is no weakening of their vowels. So start, for example, always has ɑː; stop always has ɒ; best always has e; and worst always has ɜː, no matter how non-prominent they may be made in an utterance. Weakening has nothing to do with sentence stress (accentuation), only with word stress.
As Veronica rightly points out, the KIT vowel seems to present something of a problem, since ɪ can be either strong or weak. In bridge it is obviously strong; but in the ending -ed, as in waited ˈweɪtɪd, it is obviously weak, competing as it does with the ed sometimes used in formal singing style.
So with the FOOT vowel. I would regard the ʊ as strong in the lexical word full fʊl, but as weak in the ending -ful, e.g. beautiful. The suffix vowel usually weakens further to ə anyway, though the darkness of the l, combined with syllabic consonant formation, and now its vocalization, may make this difficult to determine. The penultimate ʊ in executive and ambulance is weak, and similarly is alternatively pronounced ə.
Words like in and it indeed present a puzzle. Do these words have a weak form, which happens to sound identical to the strong form? Or are they like on, lacking a weak form?
If I assert that the first vowel in finishing ˈfɪnɪʃɪŋ is strong but the other two weak, can I prove it? Does it matter?
It seems to me that there are two further kinds of evidence that can shed light on the issue of strong and weak ɪ.
One is to look at accents of English that have lost the distinction between ɪ and ə in weak syllables. Australian English is one such: for rabbit, where I say ɪ, Australians say ə, making it rhyme with abbot. So if I want to know whether the last vowel of armistice, my ˈɑːmɪstɪs, is strong or weak, all I have to do is look the word up in my Australian Macquarie Dictionary. There I find this word transcribed with -stəs, proving that the last vowel is weak for them. So presumably the vowel in my -stɪs is weak too (despite being derived from the second part of a Latin compound).
In Australian English it and in, strong forms ɪt and ɪn, DO weaken to ət and ən in contexts where we would expect a weak form.
The other kind of evidence comes from phonological processes that are sensitive to the presence of weak vowels. Take AmE t-voicing, or “flapping” as it is often inaccurately called. Within a morpheme it operates only where the following vowel is weak. So we get the t̬ output (= ɾ, more or less) in atom (last vowel weak) but not in latex (last vowel strong). In words such as emphatic we DO get t-voicing, which tells us that the last vowel must be weak ɪ, not strong.