be to do 表示将要做某事可以用，且表示1）按计划或安排要做的事 2）表示指令，相当于should,ought to, must, have to, This meicine is to be taken three times a day. 3) 表示 想，打算，相当于intend to , want to If you are to listen to the consert, you'd better hurry up.
用动词不定式作表语时；
be to do sth表将来时可以用；
My job is to...
My ambition is to...
My hope is to...
"...是 to do"
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