因为look forward to后面要加动名词，所以要加ing
I am looking forward to hearing from you.这是邮件或者信件最后常用语。意思是：我期待您的回音。look forward to 是一个词组。 to是介词，后跟动名词， 所以是hearing
looking forward to doing sth固定搭配，意思是期待做什么
