"You finished writing the letter."
Translation:Usted terminó de escribir la carta.
Some verbs require a preposition before an infinitive. Terminar de + infinitive = to stop doing something or to finish doing something
An accent is needed over the "o" for it to be past tense. Terminó.
Termino de escribir la carta = I finish writing the letter
Terminó de escribir la carta = You finished writing the letter.
Terminaste de escribir la carta = You finished writing the letter.
I just typed "Terminaste escribir la carta" and it wasn't accepted. I understand from the other comments that you need a "de" involved, and that tú form is now accepted, but my error message was still this:
You used the tú form "terminaste" instead of the él/ella/usted form "terminó".
Please fix the error message because I was very confused.
"Terminaste" is unacceptable for some reason, although there is no context to suggest Usted is the only acceptable answer