Using Modal Verbs

Using modal verbs in a sentence is pretty simple. For basic sentences—in the simple present tense—just remember these rules:

• Modal verbs come directly before the main verb except for in questions.

• With modal verbs, use the infinitive form of the main verb. With most but not all modal verbs, to is dropped from the infinitive.

So if you want to brag about your ability to eat an entire pizza, you use the modal verb can before the infinitive form of eat without to—which is simply eat. The rest of the sentence continues as normal.

Example: I can eat an entire pizza.

If you want to communicate that circumstances are requiring you to eat an entire pizza, you might use the modal verb have before the infinitive form of eat, retaining to:

Example:  have to eat an entire pizza.

For yes/no questions, you still use the infinitive form of the main verb, but the order is a little different: [modal verb] + [subject] + [main verb infinitive].

Example: Can you eat an entire pizza?

Example: Do you have to eat an entire pizza?

Note that in the second example above, because have is a verb that only sometimes functions as an auxiliary verb and at other times functions as a main verb, the question is formed with the auxiliary verb do at the beginning.

Because modal verbs deal largely with general situations or hypotheticals that haven’t actually happened, all of the core ones can refer to present and future time but only some of them can refer to past time, and most of the time they do not change form to make different tenses. However, all of them can be used with different conjugations of a sentence’s main verb to refer to present or future time in different ways, so let’s talk a little about verb tenses and modal verbs.

Present tenses

We already covered the simple present above, but you can also use modal verbs in the present continuous and present perfect continuous tenses.

Present continuous

After the modal verb, use the word be followed by the –ing form of the main verb: [modal verb] + be + [verb in -ing form].

Example: I should be going.

Present perfect continuous

You can add a modal verb before a main verb in the present perfect continuous tense without changing much. However, note that the main verb always forms the present perfect continuous using have been, when appearing with a modal verb, never had been, even if the subject is third-person. The formula is [modal verb] + have been + [main verb in -ing form].

Example: She must have been sleeping.

Simple past and present perfect tenses

Using a modal verb in the simple past or the present perfect (which indicates an action that happened in the past but is directly related to the present) is a little trickier.

Only a few of the core modal verbs have the ability to refer to past time: could, might, should, and would. They do this by functioning at times as the past tense forms of their fellow modal verbs can, may, shall, and will. But keep in mind that, as we saw above, could, might, should, and would also have different senses in which they refer to the present and the future, indicating possibility, permission, request, habit, or other conditions. None of the modal verbs can be used in the past perfect, the past continuous, or the past perfect continuous tense.

Simple past

Of the main modal verbs listed at the top, only can and will can be used in the simple past. The expressions have to and need to can also be used in the simple past, when conjugated as had to and needed to. Other modal verbs use the present perfect to discuss events in the past.

Can and will use their past tense form plus the infinitive form of the main verb without to, just like in the present: could/would + [main verb infinitive].

Example: I could do a handstand when I was a kid.

Example: During exam season in college, I would not sleep much.

Present perfect

To form the present perfect using the modal verb could, might, should, or would, use the present perfect form of the main verb, which is have plus the past participle. As with the present perfect continuous, you always use have, even if the subject is third-person: could/might/should/would + have + [main verb past participle].

Example: I might have gone to the party, but I forgot.

Future tenses

Because the simple future, future continuous, future perfect, and future perfect continuous tenses of main verbs are all already formed with the modal verb will, when you want to indicate likelihood, permission, or any of the other conditions discussed above in the future, it often makes sense to do it in some other way than by adding a modal verb.

Example: They will be more likely to come over tomorrow if you give them plenty of warning.

However, there are also situations in which a modal verb other than willcan be used to talk about something in the future. In these cases, the new modal verb just replaces will in the sentence, and the main verb takes the same form it would with will..

Example: I can hang out tomorrow.

Example: Could I be majoring in law next year?

Example: They should have left by the time we get there.

Example: By twenty years from now, I may have travelled to more conferences than I care to recall.

Source: grammarly.com