must be 肯定猜测" 一定是‘
can't be 否定猜测“ 不可能是”
may be 可能是（ 不是很肯定）
如
Whose book is this I am not sure,it may be Jim's .
Whose book is this It must be John's .Look ,his name is on it .
The man can't be Mr.Green because he has gone to Beijing .