Quote:
Originally Posted by Ambarina
I forgot locking the door = I locked the door but I forgot that I had locked it.
I forgot to lock the door = I did not lock the door because I forgot.

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Exactly.
In these cases it is something you have or don't have a memory of.
I don't remember locking the door. - You are saying you don't have a memory of the locking.
I didn't remember to lock the door. - You know you did not lock it. You remember not locking it.
You could say "I didn't remember locking the door" BUT that is a very obscure case where you are in the present talking about the past when you didn't remember doing something in the even farther past.
In your case:
I usually forget to lock the door = I normally leave the door unlocked because I forget to lock it. - Here you know you are supposed to lock it, but you just can't remember to lock it before you leave or go to bed (or whatever).
I usually forget locking the door = Whenever I lock the door, I usually forget doing it. - Here you lock the door, but you can't remember locking it. This may be true if you do it in the mornings when you are really tired or your mind is very preoccupied with thought.
Hopefully all this helped. In your case, I think "to lock it" is the correct use and probably what is meant.
Quote:
Originally Posted by momangphonui
Thanks so much, Ambarina. I think many people misunderstand ing this grammar and I do, as welleither. I am teaching english but I also mistook forget+Ving.
PS:1/ I forget meeting her before---> It means you have met her before but you didnt remember.
2/ The thief got into the house because I forgot to lock the door.--->It means you didnt lock the door last night.
If you agree, pls note me! 
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Couple of corrections made above.