You've already learned that the pronoun "y" is used to mean there (See Y = There (French Adverbial Pronouns)).
Now here is another usage of "y".
How to use "y" to replace à + [thing/object/place] in French
Look at these sentences:
Elle répond à une lettre. Elle y répond.She's answering a letter. She's answering it.
Nous nous intéressons au sport. Nous nous y intéressons. We're interested in sport. We're interested in it.
Il pense à ses vacances. Il y pense.He's thinking of his holidays. He's thinking of them.
-> Note that y
can replace plural things
Nous pensons à faire nos devoirs. Nous y pensons.We're thinking about doing our homework. We're thinking about it.
In French, adverbial pronoun y can replace a group introduced by the preposition à + [thing(s)/object(s)/location(s)] - except people - particularly with verbs that require the preposition à.
In such cases, remember that y replaces the whole à +[noun group].
Case of people
The group à + [people] becomes simply their matching indirect pronoun, as follows:
Nous avons répondu à Sabrina. Nous lui avons répondu oui.We replied to Sabrina. We replied yes to her.
J'écrirai à mes cousines. Je leur écrirai bientôt.I'll write to my cousins. I'll write to them soon.
See also En can replace de + [phrase] (French Adverbial Pronouns)
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