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You have already studied in Chapter Ten the four main verb forms. These
verb forms are combined with helping verbs to create tenses other than the
simple present and simple past tenses. Some tenses describe the continuation
of action from the past to the present or the present in to the future. In
addition, some tenses describe more than one action being completed in the
past, but at different times. These are the progressive and perfect tenses.

The progressive tense is created using a form of the verb be with the
present participle of the main verb. The perfect tenses are created by
combining a form of the helping verb have with the past participle. It is
important not to confuse these tenses with the simple present and simple past
tenses. An easy way to separate them is to remember that both the progressive
tense and the perfect tenses use helping verbs. Simple tenses do not use a
helping verb.

It is important when you work with the past participle to put the -d ending
on the participle particularly for the verbs use and suppose. In addition,
make certain that you do not substitute the preposition of for the verb have
in phrases like could have, would have and should have.