Ils vs. eux
The French words ils and eux both translate to the English personal pronoun they for a masculine or mixed-gender group. However, they are not interchangeable. The key difference lies in their grammatical function: ils is a subject pronoun that performs an action, while eux is a stressed pronoun used for emphasis or in specific grammatical contexts.
Ils
A1ils is a subject pronoun. It is used when they is the subject of the sentence and performs the action of the verb. It is almost always followed directly by a conjugated verb.
Ils habitent à Paris.
(They live in Paris.)
Demain, ils iront au marché.
(Tomorrow, they will go to the market.)
Ils ne veulent pas manger.
(They do not want to eat.)
Ont-ils terminé leurs devoirs ?
(Have they finished their homework?)
Ils sont arrivés hier soir.
(They arrived last night.)
Je pense qu'ils ont raison.
(I think that they are right.)
Eux
A2eux is a stressed or disjunctive pronoun. It replaces ils when used for emphasis, after a preposition (like avec, pour, sans, chez), in a comparison, as a standalone answer without a verb, or after 'c'est'.
Je pars en vacances avec eux.
(I am going on vacation with them.)
Ce cadeau est pour eux.
(This gift is for them.)
Eux, ils aiment le chocolat, mais moi, je préfère la vanille.
(They, they like chocolate, but I prefer vanilla.)
Mon frère est plus jeune qu'eux.
(My brother is younger than them.)
Qui a parlé ? Eux.
(Who spoke? Them.)
Ce sont eux qui ont la clé.
(It is them who have the key.)
Tout le monde est parti, sauf eux.
(Everyone left, except them.)
Je ne pense pas à eux.
(I am not thinking about them.)
Summary
In short, use ils as the subject that performs an action, placed directly before a verb (e.g., Ils mangent - They are eating). Use eux for emphasis (Eux, ils aiment ça - THEY like that), after a preposition (pour eux - for them), in comparisons (plus grand que eux - taller than them), or when the pronoun stands alone (Qui ? Eux. - Who? Them.). While ils acts, eux is acted upon or stressed.