how to use this grammer

I look forward to hearing your reply.

In this sertence, forward is adv. so " to " 不是infinitive﹐因為在這裡它是一個preposition﹐所以後面要跟phrase.

however, i am not sure that is adv. can put behind main verb or this is a special usage?

i only know adv. will be put in front of main verb.

pls teach me how to use this grammer!!