Difference between should, must, can, may in a conversation and a sentence.

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    Usually, can = ability, may = possibility, should = recommendation, must = obligation. But there are many exceptions and other differences, and listing all of them would be beyond the scope of a single Question + Answer here. – FumbleFingers Nov 4 '18 at 14:57
  • Hint: When you're writing your question and have just entered the title, look at the list of "Questions that may already have your answer" before you go any further. – Hot Licks Nov 4 '18 at 15:28

Should: "Ideally, you'd do this/we'd prefer it if you did this/it's in your best interests to do this for whatever reason, but you don't have to if you really don't want to," though sometimes it carries the force of a politely-worded command/requirement rather than just a strong suggestion (the difference can be best ascertained by the speaker and listener's hierarchical relationship, if any).

Must: You have to do this, whether because it's the rules, whatever you're trying to accomplish simply won't work if you don't, or because I'm commanding you too and I'm in a position to give commands.

Can: You have the capability of doing this if you so choose. Often carries the additional force of "it won't make much of a practical difference either way" or "it's purely a matter of taste/preference."

May: Very similar to "can," though often (but not always) additionally implies the granting of permission in a scenario where permission is needed.

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