Vowels in unstressed syllables

I understand that vowels in the unstressed syllables are often pronounced /ə/ or /I/ .

My question is: how do I know when to use /ə/ and when to use /I/?

I don't know of any rules that dictate the pronunciation in that regard. I tend to believe that every language has some element that is particularly difficult to learn and with English, it's the pronunciation and spelling. Seriously, for what other language do national spelling competitions exist?

In "convenient" the final "e" is unstressed but it's pronounced as the short "i" sound as in "it," rather than a schwa; so the "ent" at the end sounds like "int" in the words "internet" and "internal."

I understand that vowels in the unstressed syllables are often pronounced /ə/ or /I/ .  My question is: how do I know when to use /ə/ and when to use /I/?

The vowels in unstressed syllables are collectively referred to as schwas, or simply as schwa.  When you see a schwa symbol in most transcriptions it is phonemic in nature, not phonetic.  That is, it stands for a great many different "unclear" or "reduced" vowel sounds.  There are, in essence, an infinite number of schwas.  They range throughout the whole area bounded by the triangle of vowels i as in kit, u as in cup, and oo as in look.  It is nearly impossible to specify the exact shade of schwa needed for any particular context, except in general terms.  I can only describe the schwas that exist in the variety of American English that I speak.

The kind of schwa needed depends somewhat on the spelling of the vowel being reduced, and somewhat on the consonant which follows.  As mentioned previously, when the spelling is i, y, or e, the tendency is to use a schwa closer to i.  When the spelling is a, o, or u, the tendency is to use a schwa closer to u.

When followed by t, d, ge, or ch, the tendency is to use a schwa closer to i.  (ticket, pivot, cabbage, spinach)
When followed by m, p, or b, the tendency is to use a schwa closer to u  (item, carob, julep)
When followed by l, the schwa closest to oo is almost exclusively used. (rebel, level, oval)

There is a mid-schwa which sounds midway between i and u.
When followed by s or n, the mid-schwa is most common.  (bonus, focus, pelican, demon, -tion, -cious, ...)
This means that either i or u or anything in between will sound normal for the unstressed vowel in these words.

Unusual words are lettuce and minute, where the unstressed u is the schwa closest to i.  The unstressed a in octave is also close to i.

CJ

The schwas are sometimes notated in dictionaries as full vowels (like the first "i" in discussion) when most people say the schwa that is closest to that full vowel.  Every editor of every dictionary makes his or her own choice about the way such cases will be symbolized.  There is no standard.  You may even find different symbols in different dictionaries.

In teaching English many teachers pass over the schwa quickly.  This is a sensible approach.  One wants to keep the students from obsessing about academic subtleties and keep them on the task of listening and imitating the sounds.  Imitation is much more important than any ability to have learned discussions on all the shades of schwa.  The important thing for most learners is to understand that unstressed syllables in English do not have fully articulated vowels, the way most other languages do.  The exact shade of schwa is never crucial to the meaning of the word, so attempts to control the schwa shading are a waste of time (especially in the early stages of study)  -- time that can be more profitably spent on other skills that will actually help the student to communicate well in English.  Stress patterns, for example, are much more important for being understood in English than are schwas.  In fact, stress patterns are more important than even the distinctions between the exact full vowels themselves!

In atmosphere, only the middle syllable is unstressed; only that syllable has a schwa.  The first syllable has primary stress; it has the full vowel "lax a".  The last syllable has secondary stress; it has the full vowel "tense e".  Syllables that take secondary stress are not normally pronounced with a schwa.

CJ