What's the difference between these two sentences?
I often forget to take letters to the post office.
I often forget taking letters to the post office.
In the first sentence, I often don't take letters to the post office. In the second sentence, I take the letters, but forget the experience. This is an example of how certain verbs' meaning can change depending on the form of the following verb. Other verbs take only the 'ing' form or the infinitive.