does "y" *HAVE* to replace a noun following "à"

Question

Je réponds à la lettre. Tu ________ réponds aussi.I used "la" rather than "y," but that was considered wrong.  How do I know when to use which?

Accepted Answer

Bonjour Rebecca,
Take a look at the links below:
y vs le
y with people
Remember that with "verbs that require the preposition à, y can replace a group introduced by the preposition à + [thing(s)/object(s)/location(s)] - except people".

Je réponds à la lettre -> J'y réponds  -> y = thing/object - indirect object
Je vais à Paris -> J'y vais  -> y = location - indirect object

Attention:

Je parle à Marie -> Je lui parle -> lui = person - indirect object
Je mange la pomme -> Je la mange  -> la = thing/object - direct object

See here: https://french.kwiziq.com/revision/grammar/when-to-use-le-la-l-or-les-to-replace-nouns-direct-object-pronouns  -  https://french.kwiziq.com/revision/grammar/when-to-use-lui-and-leur-to-replace-specific-people-with-him-her-and-them-indirect-object-pronouns  -  https://french.kwiziq.com/revision/glossary/pronoun-type/french-object-pronouns
I hope this is helpful.
Bonne journée !

Answer

In your example you cannot use la and need to use y. This is because la only replaces a direct object, but répondre à requires an indirect object.

does "y" HAVE to replace a noun following "à"

does "y" HAVE to replace a noun following "à"

does "y" HAVE to replace a noun following "à"

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