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1.He can swim in the river.(we know)
We know (that)he can swim in the river.
2.We can make our street more and more beautiful.(I am sure)
I'm sure we can make our street more and more beautiful.
3.Li Ming's mother may be a teacher.(he often says)
He often says Li Ming's mother may be a teacher.
4.You can attend the party.(the host said to me)
The host said I could attend the party.
5.You must hand in your homework tomorrow.(the teacher told him).
The teacher told him that he would have to hand in his homework the next day.
或：The teacher told him that he must hand in his homework tomorrow.

---绝对正确！

we know he can swim in the river

i'm sure we can make ....

he often says li ming's mother may be a teacher

the host said to me I(we) could attend the party

the teacher told him to hand in his homework tomorrow

1.We know he could swin in the river.
2.I am sure we could make our street more and more beautiful.
3.He often says that Li Ming's mother may be a teacher.
4.The host said to me that I could attend the party.
5.The teachet told him he must handed in your homework tomorrow.