to be是be动词的不定式形式.这种形式可以
1.合成谓语.例如It is going to be a fine day tomorrow.其中is going to be是合成谓语,解释为看样子会是.He used to be a naughty boy.其中used to be是合成谓语,解释为过去曾经是.
2.可以反串名词形容词副词,充当主语,宾语,补语,定语,状语
To be a student is the first period for you.主语
I want to be a student.宾语
To be here in time,he will come by air.状语
The first thing for you is to be a student.补语（也有叫表语的）
It is your job to be a man.定语
