Si una frase contiene un infinitivo y si la frase que sigue o una frase subordinada se refiere a este infinitivo se puede evitar la repetición si se usa sólo la preposición to en vez de toda la expresión. Esto se hace muy a menudo con los verbos to want, to wish, to like, to love, to hate, to hope, to try, to mean, to be going, to be allowed, to be able, to have, to need, to ought to, to need to.

Ejemplos
A: Why didn't you call me last night?
B: I tried to but there was no answer.
    He didn't want to move to Munich, he just had to because of the job.
A: Did you post the letter as I asked you?
B: No, but I was just going to.