Sounds and letters - two different things in English

▪ The letter of the alphabet 'I' is pronounced /aɪ/ in many words.  

 e.g. primary, assignment → /aɪ/       

But most of the time it is pronounced /ɪ/.  

  e.g. physics, English  → /ɪ/      

When you have a combination i+consonant+vowel, you pronounce it /aɪ/.   

  e.g. smile, strive

That is why you have to double the final letter in comparative forms not to change the pronunciation: e.g. big → bigger
e.g.        hid→ /ɪ/             hide → /aɪ/
             sit→ /ɪ/                site→ /aɪ/
             win → /ɪ/            wine → /aɪ/

When a word ends in -igh, -ight, -ign, ind, it is pronounced /aɪ/. (except wind) 
e.g. rewind, plight, ...
▪ If you add 'r' after the 'i', the pronunciation is different again, as you pronounce it /ɜː/   

  e.g. bird → /ɜː/
 or even /aɪə/
 fire →/aɪə/
When you have a combination i+consonant+vowel, you pronounce it /aɪə/
e.g. fir → /ɜː/    fire →/aɪə/