Close front vowels:

We find [i] occurring:
1. In word-final position in words spelt with final ‘y’ or ‘ey’ (alter one or more consonant letters), e.g. ‘happy’, ‘valley’, and in morpheme-final position when such words have suffixes beginning with vowels, e.g. ‘happier’, easiest’, ‘hurrying’.
2. In a prefix such as those spelt ‘re’, ‘pre’, ‘de’ if it precedes a vowel and is unstressed, for example in ‘react’, preoccupied’, ‘deactivate’
3. In the suffixes spelt ‘iate’, ‘ious’ when they have two syllables, e.g. ‘appreciate’, ‘hilarious’
4. In the following words when unstressed: ‘he’, ‘she’, ‘we’, ‘me’, ‘be’ and the word ‘the’ when it precedes a vowel.

In most other cases of syllables containing a short close front unrounded vowel we can assign the vowel to the /I/ phoneme, as in the first syllable or ‘resist’, ‘inane’, ‘enough’, the middle syllable of ‘incident’, ‘orchestra’, ‘artichoke’ and the final syllable of ‘swimming’, ‘liquid’, ‘optic’. It can be seen that this vowel is most often represented in spelling by the letters ‘I’ and ‘e’
Weak syllables with close back rounded vowels are not so commonly found. We find [u] most frequently in the words ‘you’, ‘to’, ‘into’, ‘do’, when they are unstressed and are not immediately preceding a consonant, and ‘through’ and ‘who’ in all positions when tey are unstressed. This vowel is also found before another vowel within a word, as in ‘evacuation’, ‘influenza’
(from ROACH, P. (2005) English Phonetics and Phonology. CUP. Cambridge)