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A: There's somebody ringing the doorbell.

B: That will be Bob. He said he would be here at this time. / That will be the postman at the door now.

May I ask how do we negate these two sentences? Can we use "can't" or "won't"? Like:

A: There's somebody ringing the doorbell.

B: That won't/can't be Bob. He said he wouldn't be here at this time. / That won't/can't be the postman at the door now.

Please enlighten me on this? Thank you for your opinion.
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You can negate them as you say, yes. I think that most of the time, however, people use will to guess who is there; they don't as often use won't to declare who is probably not there!
Be aware that many of the expressions you have been studying recently have just one or two typical uses. It's all right to learn how to negate them, how to use them in the past, or how to form questions from them, but you need to realize that not all of these forms are equally useful in real conversations. Emotion: smile
CJ
Comments  
Thank you for your advice! And your answer is very helpful to me. Thank you!