As-tu repondu a la lettre se ta soeur?

How would I know whether to use indirect object pronoun or the pronoun y?

J'y ai repondu.
or
Je lui ai repondu.

Sorry i don't know how to do the accents.

3 answers

If you have a PC (not a laptop) then accents are simple.
While holding down the Alt-key press the following numbers on the NUMERIC keyboard:
à = 133
â = 131
é = 130
è = 138
ê = 136
ë = 137
ô = 147
î = 140
ç = 135
Thank you for using the Jiskha Homework Help Forum. If you have a MAC, accent marks are even easier . If the chart Anonymous gave you doesn't work, let us know if you have Windows or not.

That is probably "de" ta soeur? Because you are replacing "à la lettre" your first choice is correct: J'y ai répondu.

If it were "As-tu répondu ta soeur?" Then it would be "Je lui ai répondu." It's when giving an answer to a letter, a question, etc. that it becomes "répondre à."

Mme
could it also be "Je l'ai repondu." Using direct object pronoun?