Just, yet, still, already, again. English Grammar. - learn English,grammar,english
English infographic Modals: grammar for the esl classroom
All modal verbs are auxiliary verbs, which means they can only be used with a main verb. Modal verbs cannot be a main verb. They are used to express ideas such as possibility, prediction, speculation, deduction and necessity.
Shades of Modality [infographic]: "Even if a reader has no idea what a modal verb is (a type of auxiliary verb that expresses possibility, necessity or obligation), this statement is more likely to agitate than inform, and it would start any other article on a sour note. [...] Because of their commanding tone, modals must be used carefully, and some writers attempt to avoid them entirely."
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Using English Modal Verbs to make requests. This link has a situation table for using modal verbs.