英语不定式作宾补和不定式作状语的区分?

The teacher ask the students【 to close the windows宾补,由于是宾语the students的动作】
he boy needs a pen 【to do his homework目的状语】如句子变成：
To do his homework,the boy needs a pen.
宾补就是宾语发出的动作..
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