I think he is on duty today.为什么用ON,可以去掉吗

I think he is on duty today.为什么用ON,可以去掉吗 I don't think he is on the first floor.反义疑问句 I think he is sure to___(succeed). i donot think he who is right I think he is must abolish. I don't think he will come on time -------A won't B will be C does D IS he I don’t think he will come here on time,________?A.won’t he B.will he C.does he D.is he I ______ (think) he is a clever boy ,is he I don‘t think he is tall,_____?He doesn't think he is tall,_____? I think he is a clever boy同义句I think he is a clever boy.I think him . He ___ fast,I think he is good____.(run) I think he is tall he 为什么不用宾格? I think he is busy today 同义句 I ( ) think he ( )( ) today I don't think he is right.为什么不能是 I think he isn't right. I don't think he is .能否说成I think he isn't . Lei Ming is strong,I think he is s____ I don't ()with peter.I think he is wrong. 改句子,改为同义句He always walks to school on weekends.（同义句）He always gose to school ( ? ) ( ? ) on weekends.I think think that man is tall.（同义句）I ( ? ) ( ? ) that man ( ?) ( ).