The letter i has two main sounds because syllable structure controls it. Its name, the sound you say when reciting the alphabet, is /aɪ/, and that is also one of its two in-word sounds. In a closed syllable, where a consonant closes in after the vowel, i usually takes the short sound /ɪ/. In an open or silent-e pattern, it usually takes /aɪ/, which matches its name.
Walk through one case: sit has the vowel closed in by t, so the sound stays short /ɪ/. Add a silent e and you get site. Same t, same single syllable, but the e makes i say its name, /aɪ/.
A quick analogy: think of the consonant as a door. When the door shuts right after the vowel, the sound is trapped and shortened. Prop it open with a silent e, and the sound stretches into /aɪ/.
The same word, two doors
One pattern worth adding is the open syllable, where the syllable ends right at the vowel with no consonant closing it in. Because nothing shuts the vowel off, i usually takes its long letter-name sound /aɪ/.
Take the word rising. Break it into syllables and you get ri-sing. The i sits at the end of the first syllable, so that syllable is open, and the vowel stretches into /aɪ/. The same syllable split explains timer, where ti- opens the word and the i says its name.
Compare that with a closed syllable like rip. Here the p follows the vowel and closes it in, so the sound stays short /ɪ/. Whether a consonant comes right after the vowel, or the vowel stands alone at the syllable end, is the causal difference between the two sounds.
One caveat: the open syllable still lets in exceptions such as police, where the i sits in a spot that pattern-spotting alone would not predict, and it takes the short /ɪ/ sound rather than the expected /aɪ/. So the syllable split gives you a strong first guess, but a dictionary check settles the leftover few.
When the pattern breaks
The pattern helps you guess, but treat it as a tendency, not a rule. English spelling often does not match pronunciation, so exceptions exist even when the syllable looks familiar.
Give looks like the silent-e pattern, yet it takes /ɪ/ rather than /aɪ/. And police has i in a spot where no simple pattern would lead you to expect its sound: it takes the /ee/ sound you hear in machine or taxi, so there is nothing in the spelling itself that signals it. These are words you simply learn by listening.
So the letter i mostly sticks to two main sounds, the short /ɪ/ of sit and the long /aɪ/ of time, and the spelling pattern tells you which one to expect. Keep the two-main-sounds framing in mind, and treat the exceptions you meet as boundary cases worth noticing.
So the syllable pattern is a reliable starting guess: closed-in vowel suggests /ɪ/, open or silent-e suggests /aɪ/. But a few words keep surprising sounds, so checking a dictionary is the safe move.