Regular verbs consist of three main parts: the root form (present), the (simple) past, and the past participle. Regular verbs have an -ed ending added to the root verb for both the simple past and past participle. Irregular verbs do not follow this pattern, and instead take on an alternative pattern.
Some verbs "help" out a main verb, adding more info about tense or mood. We call these helping verbs or auxiliaries. Helping verbs must always accompany another verb. For example:
He is driving home. (is works with driving to say it's happening now)
The helping verbs be and have help express different tenses and the helping verb do helps express negation or emphasis. All three of these verbs can help to form questions too.
I have seen ghosts. (have helps form the present perfect tense)
I do not have a pencil. (do helps negate the verb have)
Do you have a pencil? (do is only here to help form the question)
Modals are a subgroup of helping verbs that help express certainty, possibility, or obligation. Modal verbs include can, will, could, would, should, may, might, and must. Other verbs that can act like modals include used to and have to.
I can eat my dinner. (the main verb is eat, but can expresses possibility)
We have to leave. (have to adds obligation to the action of leaving)
Helping verbs are often shorted to form contractions in English, but you should avoid using contractions in most formal writing. Helping verbs also have irregular forms - see the box on Irregular Verbs for more info.
Verbs help tell the time of an action by taking different forms.
The simplest form is the present tense. Verbs in the present tense describe an event currently happening:
For example: I listen carefully in class. (listen is in the present tense)
The past tense describes events that already happened:
For example: I listened carefully in class. (the -ed ending changes the verb listen to the past tense)
Using the helping verb have (or has) we can form the present perfect tense:
For example: I have finished all my homework. (At this time, I have already finished it)
And had is used for the past perfect tense:
She had escaped when the police arrived. (In the past when they arrived, she was already gone.)