The most common auxiliary verbs are "be," "do," and "have", and you may also use these verbs on their own. Other common auxiliaries are "can," "could," "may," "might," "must," "ought," "should," "will," and "would." A verb like these is called a modal auxiliary and expresses necessity, obligation, or possibility. Modal auxiliary verbs like ‘can’ and ‘should’ usually occur with main verbs e.g. ‘can pay’, ‘should pay’. They add meanings like possibility and obligation to the main verb
The use of tenses in English may be quite complicated, but the structure of English tenses is actually very simple. The basic structure for a positive sentence is: Subject + auxiliary verb + main verb
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