何時在to之後用ing form?

If TO is used as a preposition, not a part of the infinitive, and followed by a verb, you must say TO -ING.

eg. I prefer driving to travelling by train.
Are you looking forward to seeing Ann again?
She is used to driving on the left.

Situation 1)
look forward to ____ing
Example : I look forward to hearing from you.

Situation 2)
used to ___ing
Example : I am used to driving on the left.

I look forward to seeing you.
I prefer swimming to running.
I admit to stealing his pencil.

there are lots of other cases
actually if the "to" is a preposition, the word before it will be in ing form or a noun.