There are two vowels that are commonly found in weak syllables. These two vowels are ‘i’ and ‘u’.
Very long time ago, when I had the first contact with English phonemic chart, I was taught about long vowels such as /i:/ and /u:/ (the colon symbol shows us that the vowel is a long one). It is not the case in our days since this classification is obsolete.

That being said, there were three kinds of ‘i’ vowels: /ɪ/ as in the word ‘fit’, /i:/ as in the word ‘eat’, and /i/ as I see it in the word ‘busy’ (/ˈbɪzi/). It is not easy to distinguish between /i/ sound in the word ‘eat‘, and /i/ sound in the word ‘busy’ but for me the /i/ in ‘busy’ is shorter, weaker, and shares characteristics from the other two vowels (/i:/ and /ɪ/). It is the same in the case of /u/ vowel.

The picture above shows us how two famous dictionaries (dictionary.com and dictionary.cambridge.org) show different phonemic transcriptions (or maybe I should say different symbols). The difference appears in the words ‘eat’ and ‘busy’. The Cambridge dictionary (on the right side) uses /i:/. Dictionary.com uses a normal /i/. So, Cambridge differentiate the /i/ sound in the word ‘eat’ from the /i/ sound in the word ‘busy’ as being /i:/ and /i/ respectively.

Native speakers of English, and also learners of English, seem to feel comfortable with the simplified transcription of dictionary.com which is a strong argument in its favor.

Let’s take a look at where these vowels are found. We find /i/ occurring in the following situations:

• We find /i/ in final position in words spelled with final ‘y’ or ‘ey’ (example: happy, valley etc.)
• We find /i/ in morpheme final position when such words have suffixes beginning with vowels (examples: happier, easiest etc.)
• We find /i/ in a prefix such as those spelled ‘re-‘, ‘pre-‘, ‘de-‘ if it precedes a vowel and it is unstressed (examples: react, preoccupied, deactivate etc.)
• We find /i/ in suffixes spelled ‘-iate’ and ‘-ious’ when they have two syllables (examples: appreciate, hilarious etc.)
• We find /i/ in the following words when unstressed: ‘he’, ‘she’, ‘me’, ‘we’, and ‘be’.
• We find /i/ in the word ‘the’ when it preceeds a vowel.

In all other cases (or most of them) of weak syllables containing a close vowel (unrounded) we can assign the vowel to the /ɪ/ phoneme (examples: resist, enough, incident etc.) This vowel is often represented in spelling by the letter ‘i’.

Weak syllables with close back (rounded) vowels are not found very often. We find /u/ in the words ‘through’ and ‘who’ when they are unstressed. We also find /u/ in the following words: ‘you’, ‘to’, ‘into’, and ‘do’ when they are unstressed and are not immediately preceding a consonant. Within a word, this vowel is also found before another vowel (example: evacuation).