I mean why الا instead of اَلْ for al. Because, for example, الْمَدِنة the city is written with اَلْ and not الأ. Also لا is supposed to mean laa and not only l, right? So writing الأُسْتاذ should be pronounced like alaa-ustadh. That is my question, and I was wondering if you write الأ just in case the next word starts with a vowel (ustadh).
Well, the word originally is:
The Alif-Hamza in the beginning is part of the original word.
If we want to define the word (THE professor) then we add "AL" الـ and it becomes:
Which is pronounced as Al-Ustadh.
The ligature لا or لأ is simply Lam combined with Alif. When Alif comes after Lam, it would be cumbersome to write it as الـأ - so this ligature or shape is used, whenever, wherever, Alif comes AFTER Lam.