The letter i can be pronounced in different ways. For example, when you say “nice” the sound is /ai/, whereas when you say “sit” the sound is /i/.
Let’s have a look at the sound /ai/ first. This sound is a diphthong, what means that is formed by two different sounds. Watch this video and see how it is pronounced.
This diphthong is also frequently spelt “ie” (die), “y” (cry) and sometimes “igh” (high) or “uy” (buy).
The sound /i/ is what we call a short vowel, that is, it only has one short sound. Watch this video and see how it is pronounced.
This sound is mainly spelt with “i” (win) and “y” (gym). However, it can have other spellings when it takes place in an unstressed syllable (decided, village…).