（1）be used for doing sth. 意为“（被）用来做某事”，强调用途或作用，相当于be used to do sth.
Knives are used for cutting things.
=Knives are used to cut things.
刀子被用来切东西。
（2）be used as … 意为“(被)用作……”，强调被当作工具或手段来用。
Mobile phones are used as tools which make people communicate easily.
手机被用作人们之间轻松交流的工具。
(3)be used to do sth. 意为“(被)用来做某事”，强调目的。
Computers can be used to do many things.
电脑可以用来做许多事。
(4)be used in … 意为“被用在……里”。
The hat was used in the play in the past.
这个帽子在过去被用在戏剧中。
Let's have a try.
谢谢！