1. What Are Modal Verbs?

Present continuous
After the modal verb, use the word befollowed by the –ing form of the main verb. 
[modal verb] + be + [verb in -ing form]
I should be going. 
Present perfect continuous
You can add a modal verb before a verb in the present perfect continuous tense without changing much. However, when using a modal verb, you must always use “have,” never “had,” even if the subject is third-person. 
[modal verb] + have been + [verb in -ing form]
She must have been sleeping. 
Past tenses and present perfect
Putting a modal verb in the simple past,past continuous, and present perfecttenses is a little trickier. 
For starters, two modal verbs in particular have a simple past tense: can and will. If you want to use either of those in any of the past tenses, you must first conjugate them into their past-tense form: 

• 
  can -> could
  
• 
  will -> would
  

All the others remain the same, although some can’t be used in the past at all. Modal verbs often deal with hypotheticals, but if an action already happened in the past, it can’t be hypothetical. These are mostly for speculating about the past, such as wondering “what if . . .”
None of the modal verbs can be used in the past perfect or past perfect continuous. 
Simple past
Of the main modal verbs listed at the top, only can and will can be used in the simple past. Have to and need to can also be used, as long as they’re conjugated accordingly 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 + [verb in infinitive]
I could do a handstand when I was a kid. 
During exam season in college, I would not sleep much. 
Past continuous
Again, only can and will can be used in the past continuous. It’s formed just like the present continuous, except with the past form of the modal verb. 
could/would + be + [verb in -ing form]
I could be working right now. 
Present perfect
Instead of using the infinitive form of the main verb, just use the present perfect form, which is “have” plus the past participle. As before, you must always use “have,” even if the subject is third-person. 
If you’re using can, be sure to use its past tense form of could. 
[modal verb] + have + [past participle]
I might have gone to the party, but I forgot. 
Future tenses 
The truth is that most of the future tenses already use modal verbs because they use “will.” If you want to use different modal verb, such as “can” or “should,” you can use it normally with the infinitive form of the verb, and without will. 
I can hang out tomorrow. 
Should I major in law next year?