The rules of Laam can be summarized based on the way it is represented in the Uthmaniy mushaf as two broad rules. This is regardless of its actual type. Whether it is a Laam of Nouns, Laam of Definiteness, Laam of Verbs, Laam of Command, Laam of Harf.
The Laam that is made Clear - This Laam is pronounced clearly (إظهار)without any merging. It is also referred to as Laam Qamariyyah (اللام القمرية) when it is the Laam of definiteness. It is represented in the Mushaf as a ل with a sukun symbol on it (the top part of the letter ح).
Examples are: القمر - الحكيم - السنتكم - فلينتظر – يلتقطه
The Laam that is Merged - This Laam is merged into the neighboring letter. It is also referred to as Laam Shamsiyyah (اللام الشمسية) when it is the Laam of Definiteness. It is represented in the Mushaf without any symbol on it, and the letter after it will have a shaddah on it as a result of the merging.
Examples are: الشّمس - الرّحمن - وقل رّب - هل لّك - بل لّا