Verbs like can and may are called modal auxiliaries, though we often refer to them simply as modal verbs or modals. We frequently use modals when we are concerned with our relationship with someone else. We may for example ask for permission to do something; grant permission; give or receive advice; make or respond to requests and offers, etc. We can express levels of politeness both by the forms we choose and the way we say things. The bluntest command (You must see a doctor), with certain kind of stress, might be more kindly and persuasive than the most complicated utterance ( I think it might possibly be advisable for you to see a doctor).
Modals of Advice Board Game

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