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American English uses modals as helping verbs. They are used often in writing and conversation. The verbs [be], [do], and [have] are primary helping verbs. Modal verbs are secondary helping verbs.
Use Modal Verbs to Express Politeness
Use models to ask permission. can, could, or may
Example: Can I use your telephone? Yes, you can. May I be excused? Yes, you may.
Use Modals to Express Shades of Meaning from the Speaker’s Point of View
Words may be similar but differ slightly. Modals can convey a wide range of meaning from positive to negative.
how certain it is something will happen could can, could,
obligation and necessity must, should
ability and inclination can, may, might, will