Il vs. lui
The French words il and lui can both be translated as he or him, but they are not interchangeable. The key difference lies in their grammatical function within a sentence: il is a subject pronoun, while lui is an indirect object pronoun or a stressed pronoun.
Il
A1The word il is a subject pronoun. It is used when he is the person or thing performing the action of the verb. It almost always comes directly before the verb.
Il mange une pomme.
(He is eating an apple.)
Il est très intelligent.
(He is very intelligent.)
Il va arriver en retard.
(He is going to arrive late.)
Demain, il travaillera à la maison.
(Tomorrow, he will work from home.)
Aime-t-il le cinéma ?
(Does he like the cinema?)
Il ne veut pas venir avec nous.
(He does not want to come with us.)
Lui
A2The word lui has two main functions. First, it is an indirect object pronoun, meaning to him or for him; it receives the action of the verb indirectly. Second, it is a stressed pronoun, used for emphasis or in specific grammatical contexts, such as after a preposition (e.g., avec, pour, sans).
Je lui donne un conseil.
(I am giving a piece of advice to him.)
Elle lui parle tous les jours.
(She speaks to him every day.)
C'est un cadeau pour lui.
(It is a gift for him.)
Je pars en vacances avec lui.
(I am going on vacation with him.)
Mon frère est plus jeune que lui.
(My brother is younger than him.)
Qui a appelé ? C'est lui.
(Who called? It's him.)
Lui, il préfère rester à la maison.
(He, he prefers to stay home.)
Sa sœur et lui vont au concert.
(His sister and he are going to the concert.)
Summary
In short, use il when he is the subject performing the verb's action (He speaks). Use lui when the meaning is to him or for him (I speak to him), or when him is used for emphasis or after a preposition like with, for, or than (with him, taller than him).