What is the difference between primary and main auxiliary verbs

Dear Student, 

• Primary Auxiliary Verb: Primary Auxiliary verbs are Be, Have and Do. They change their form according to the number and person of the subject. They have infinitive and participle forms.

                                   1.   The verb 'be' forms the passive voice and the continuous tense.
                                   2.   The verb 'have' forms the perfect tenses.
                                   3.   The verb 'do' enables us to negate or to ask questions.

• Modal Auxiliary Verb: The verbs can, could, may, might, will, would, shall, should, must etc. are called modal auxiliary verbs. These verbs cannot be used alone. A principal verb is either present or implied. They do not change their form, whatever be the number and person of the subject. They do not have infinitive or participle forms.

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