The most common auxiliary verbs are "be", "do" and "have", and you can use these verbs on your own as well. Other common auxiliaries are "can," "could," "may," "might," "must," "must," "should," "will," and "would." A verb like these is called modal and expresses necessity, obligation or possibility. Modal auxiliary verbs like 'can' and 'should' usually occur with main verbs, eg 'can pay', 'should pay'. They add meanings such as possibility and obligation to the main verb.