Ⅰ. be used for “(被)用来做……”,强调用途或作用。
Ⅱ. be used as “(被)作为……而用”,强调被当作工具或手段来用。
Ⅲ. be used by “被……使用”,by后跟人物,强调使用者。如:
① Keys are used for opening locks. 钥匙是用来开锁的。
② “Swim” can be used as a noun. “Swim”可作名词用。
③ Recorders are often used by English teachers. 英语教师经常使用录音机。
④ Wood can be used for making paper. 木材能用来造纸。