You can watch fluent forever series about Arabic pronunciation on youtube, I think it'll help you I'll try to explain it : alif letter has three different pronunciation according to what is written on it or below it, for example: in اثنان word, grammartically written as اِثْنَانِ so in this case the alif letter has what we called kasrah below it so we pronounce this alif as the i letter in (stupid ) word and this is its ipa /ˈst(j)upɪd/, you can find that the kasrah is written if you use a dictionary called (almanny), It's online and free