i am looking forward to hearing from you!这里的TO后面应该是用动词原形hear才对啊,为什么用hearing呢

look forward to doing固定用法
帮你总结下,to后面加doing的固定用法还有：
pay attention to doing
devote...to doing
make contributions to doing
be used to doing
turn to doing(转行做.)
get down to doing (着力于做某事)
come close to doing
on one's way to doing(在成为.的路上)
reject to doing
stick to doing
be addicated to doing