when to use lui and when to use il, while both has the same meaning which is "he"

Question

Answer

Voici Alain. Il est acteur. (This is Alain. He's an actor. He = subject pronoun)

Je suis professeur. Lui (, il) est acteur. (I'm a teacher. As for him, he's an actor. Lui = stressed pronoun = as for him)

"Moi, toi, lui, elle, nous, vous, eux, elles" are called stressed (or tonic) pronouns.

We use them when we want to insist on whom we are talking about. 
Qui a fait ça ? (Who did that?)
C'est lui. (It's him/He did.)

We also use stressed pronouns after prepositions, e.g.: 
Ce cadeau est pour lui. 
Ce livre est à moi. 
La fête a lieu chez toi. 
C'est une photo d'eux.
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