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 做真正的主语那用it , it's 都可以吗？可以的