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Tom should know the address. Tom may know the address. In case of expressing possibilities can should and may be interchangeably used? or Is there any striking difference between them?

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  • It might help to note that Tom should know the address, but he might [or may] not is a perfectly natural utterance, where if should is given heavy stress, the implication is that Tom is expected to know it (it's his "duty" to know things like that). If there's no stress, it just means he might know, or he might not. But reversing the two auxiliaries to give Tom may know the address, but he should not [more naturally, shouldn't] is a fairly unusual construction that would always mean it's improper/illegal for Tom to have that information. Dec 21 '18 at 15:07
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Tom should know the address.

When used to refer to possibility, this statement means there's a good or decent chance that Tom knows the address. There is good reason to think he does know it.

Tom may know the address.

This statement is neutral. There is a chance that Tom knows the address.

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  • It could also mean that it is expected of Tom to know the address, regardless of the whether he actually does know it, for example as a requirement of his job.
    – Tashus
    Dec 21 '18 at 15:51
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    @Tashus: Yes, but my answer explicitly limits the context: "When used to refer to possibility". Dec 21 '18 at 16:13
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"Tom may know the address" expresses the possibility of Tom's knowledge of the address.

"Tom should know the address" expresses, in a polite way, the obligation that Tom has to know the address. However, it is worth mentioning that in spoken English, this is also used to express possibilities.

The speaker guesses that Tom knows the address when they use "Tom must know the address".

thanks and enjoy learning, Travis

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  • I suggest that Tom should know the address expresses NOT the obligation, but the likelihood of his knowing. Dec 21 '18 at 14:04
  • You are also right, however, in that case use of must is a better choice as mentioned. Dec 21 '18 at 15:09
  • @RonaldSole It can express obligation, or at least a failed expectation. For example, if Tom wrote the wrong address, one might say "Tom should know the address" to express disappointment that he has not yet learned it correctly. Obviously it is not likely that he knows the address in this context, since he wrote it incorrectly.
    – Tashus
    Dec 21 '18 at 15:49
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In speech, the meaning of should can change depending on where emphasis is placed. With emphasis on 'should', doubt or uncertainty is being expressed:

Tom should know the address. Tom probably knows the address, I'm not sure.

With emphasis on the verb after 'should', impatience or surprise is being expressed:

Tom should know the address. Why is he asking what the address is? I've told him a hundred times!

"May" does not carry any such implications, although if said in a doubtful tone, can express uncertainty.

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