1、Sth . beused to do sth.(是固定词组，意思是用来做某事。)
2、Perfomatives: are the sentences that did not state a fact or describe a state, and are not verifiable. But they are used by the speaker to do STH.(既不是陈述事实也不是描写状态，无法证实，但被说话者用来做事的。)
used to do sth