Hi DiemQuynh, I would argue that your trouble with this sentence is not your fault, but the fault of a poorly designed test question. 1. Have you forgotten to lock the back door frequently? This sentence is actually okay without any corrections. It does sound strange at first, but there are contexts when it would be okay. For example, if you feel like you are becoming forgetful, you might go and see the doctor to tell them about it. As an example, you tell the doctor "Yesterday I left the house without locking the back door." Your doctor, wondering if this was a one-off occurrence or a common occurrence might ask "Have you forgotten to lock the back door frequently?" 2. Have you forgotten locking the back door frequently? This sentence is also fine, but the meaning is completely different. It also sounds very awkward at first. But imagine this: you lock the back door, and then go out to work. But on your way to work, while you are driving, you think to yourself "Oh no! Did I lock the back door?" You DID lock the door. But you can't remember locking it. You have forgotten locking it. If you mention this incident to your doctor, they might ask "Have you forgotten locking the door frequently?" Continued in comment (my response was too long!)...