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 It is not the case in our days since this classification is obsolete.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:
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).