There are 2 rules:
Firstly: if there are fathah or dammah before ل in the word اللَّهُ, then ل is read with a full mouth.
Secondly: if there is kasrah before ل in the word اللَّهُ, then ل is read with an empty mouth:
مَا شَاءَ اللَّهُ
ma shaa LLah
Find the rule of the letter ل in the word اللَّهُ in surah an-Nasr:
In surah al-Bayyinah:
How many rules of lam are there in the word اللَّهُ?