used to do sth. 过去常常现在不了
eg: I used to eat so much meat, now I love fruit then it.
used to sth./used to doing sth. 习惯了(to是介词了,接名词或动名词)
eg: I used to getting up early every morning.
be used to do sth. 被用于(被动语态)
eg: Wechat is used to be the most common instant communication client software nowdays.
原文:http://blog.51cto.com/lander/2083269