be動(dòng)詞后面的動(dòng)詞形式有三種可能：
-ing:現(xiàn)在進(jìn)行時(shí),I'm trying to give you the right answer.
-ed:被動(dòng)語(yǔ)態(tài),I'm told that you don't know her.
to do:
1、表示按計(jì)劃或安排要做的事。例如：They said goodbye,little knowing that they were never to meet again.他們告了別,不知道以后再也不會(huì)見面了。
2.表示“應(yīng)該”,相當(dāng)于should,ought to.例如：You are to report to the police.你應(yīng)該報(bào)警。
3.表示“必須”,相當(dāng)于must,have to.例如：The letter is to be handed to him in person.這封信必須親手交給他。
4.表示“想,打算”,相當(dāng)于intend,want.例如：If we are to be there before ten,we’ll have to go now.如果我們要在十點(diǎn)前到,我們現(xiàn)在就得走。
5.用于第一人稱疑問句,表示征求對(duì)方意見。
6.用于否定句,表示“禁止”,相當(dāng)于mustn’t.例如：You are not to smoke in the reading-room.你不可以在閱覽室里吸煙。
7.表示“可以,可能”,相當(dāng)于may,can.例如：The news is to be found in the evening paper.這條消息可以在晚報(bào)上見到。
8.were to do sth.用于if或even if / even though從句中,表示對(duì)未來(lái)的假設(shè)。
9.be to blame(該受責(zé)備,對(duì)某壞事應(yīng)負(fù)責(zé)任)與be to let(待出租)兩種結(jié)構(gòu)中，用不定式的主動(dòng)形式表示被動(dòng)含義。例如：This house is to let.這房子要出租。