（A）He may not be there.
(B) He can't be there.
这两句话的意思相同吗？
这两句话的意思是完全不同的。在 can not 结构中, 否定词 not 是否定助动词 can 的: He can't be there. = It is not possible that he is there. 他不可能在那里。类例:
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You can't be serious. = You must be joking. 你不可能是当真的。= 你一定是开玩笑。
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He can't be more than thirty. (Wells) 他不可能超过三十岁。
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If he said that, he can't be telling the truth. (Eckersley) 如果他说过这话,那他不可能说的是真话。
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She can't be awake. 她不可能是醒着的。
而在 may not 结构中, 当 may 作“可能”解时, 否定词 not 一般是否定其后的成分的。He may not be there. = It is possible that he is not there. (他可能不在那里。) 类例:
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You may not be serious. 你可能不是当真的。
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That may not be true. 那可能不是真的。
但是，当 may (might) 作“允许”解时，则 not 常常是否定 may (might) 的：
You may not go swimming today. (CGEL) 今天不许你们去游泳。
请做下面的英语试题：
The sign on the lawn says clearly that people ____ on the grass.
A. will not walk B. might not walk C. have not walked D. may not walk
(参见 may not/might not)