Using English Modal Verbs to make requests. This link has a situation table for using modal verbs.
Verb tenses with timelines - learn English,verb,tenses,grammar,english
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.
Just - Yet - Still - Already - Again. English Grammar. - learn English,grammar,english
Conditionals and IF clauses - English Grammar