They can’t have been doing their homework,能不能改成下面句子They isn't doing their homework----------------改前和改后的句子除了语气不一样外,还有别的不一样吗?

I think they were Canadian.I don't think they were.They can't have been Canadian.They must have been Australian.帮我看看有无语法错误 They can't have been doing their homework请问can't have been是对刚才所发生的事的推测吗? 用can't have been造句! Greens must have been to Shanghai ,_____they?A.hadn't B.didn't C.can't D.weren't还有haven‘t they They can’t have been doing their homework ---doing是什么词性?have been 在这里还是表现在完成时吗 They must have been enjoying themselves there,or they _____ so long.[A] can't have stayed [B]wouldn't have stayed [C] needn't have stayed [D] couldn't stay为什么选B?清翻译整句 They can't have been watching the football match this time yesterday,___?A.must theyB.can theyC.have theyD.were theywhy choose They can’t have been doing their homework--句中“can't是不是可以直接去掉,从语法上来讲没有错啊?还有They can’t have been doing their homework能改成：They haven't been doing their homework They can't have been doing thier homework.have been 是现在完成时,但是后面do为什么要变成doing?They can't have been doing thier homework.这句话 have been 是现在完成时,但是后面do为什么要变成doing?为什么不用do呢? (They must have been there yesterday ,______they?)能用weren't they 还是didn't they? 英语翻译A.can't have been B.mustn't have been C.shouldn't have been D.wouldn't have been can’t have been与couldn’t have been的区别? Where have they been? 1.She ( ) lived here for three years. 2.They can't spend that money, because it is not( ).1.A.have been B. didn't C.has been D.has2.A.theirs B.their C.they D.them can't have been和must have been每一个的用法 She can't have been Danish. and She must have been Swedish. be,must be,can't have been,must have been吗 must have been和can't have been和现在完成时有没有联系啊