to learn a foreign language well is useful,I think(改为同义句)

题目

to learn a foreign language well is useful,I think(改为同义句)
I think it useful to learn a foreign language well .
这里的第二句话中为什么用it而不是it's

答案

I think it useful to learn a foreign language well .此句中it 做形式宾语,to learn a foreign language well 是真正的宾语,useful做宾补
=I think（ that ) it'S useful to learn a foreign language well .此句中it 是it形式主语,is usefull为系表结构,to learn a foreign language well 做真正的主语