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