Related to the question of whether we must use or must avoid the use of "to" before the infinitive, is the question of the particle "to" implies the infinitive so that in idiomatic English the infinitive is not repeated. What??????
OK, if you have not yet read the question and answer about using "to" before the infinitive, please read it before continuing.
Cool. Now that you have read when we must use and may not use "to" before the infinitive, I will explain when the "to" allows you not to state the infinitive at all. An example:
"Mom, may I go to the movies with you and Dad?" asked Sarah.
"No, you may not," replied Sarah's mother sternly."
"But why not?" asked Sarah pleadingly. "Pleaaaaase, Mom?"
"Because you're not allowed to," responded Sarah's mother.
Sarah's mother does not answer, "Because you're not allowed to go to the movies" because the particle "to" implies that she is not allowed to do what she asked permission to do, namely, go with her parents to the movies. Another example:
"Why didn't you dance with John at the party?"
"Well, he never asked me to."
Again, there is no need to repeat the infinitive "dance" in the answer because "dance" is implied by the particle "to." A third example:
"Why didn't you bring home pizza? I told you I wanted pizza!"
"Oh, I'm sorry. I forgot to."
Also in this dialog, the particle "to" implies the infinitive "bring" in the response.
Do we ever include the infinitive after the "to" in the answer? Yes, we sometimes include the infinitive for emphasis. For example:
"I want to eat pizza tonight."
"Yuck, I don't want to eat pizza. You know I hate pizza. Let's get Chinese food instead."
In summary, we don't have to and usually don't repeat in the response to a question or previous statement the infinitive contained in the question or statement. We say that in the response the infinitive is implied by the particle "to." Sometimes, though, we repeat the infinitive for emphasis or to make a point -- like the "you know I hate pizza" in the example dialog.