Modal forms

Can / Be Able To
Below are examples of can / could / be able to for ability and permission

Examples
a. He can play tennis well.
b. She is able to speak five languages.
c. They can come on Friday.
d. Jack will be able to come next week.

Usage
Use ‘can’ or ‘be able to’ to express an ability or possibility
✦The future of ‘be able to’ is ‘will be able to’

Example
He could swim when he was five.

Usage
Could in the past means the general ability to do something.

Examples
a. They were able to get tickets for the concert.
b. I was able to finish before 6.
c. I couldn’t come last night, sorry. -OR- I wasn’t able to come last night, sorry.

Usage
If someone was in the position to do something, or managed to do something, we use was/were able to instead of ‘could’
✦ In the negative, ‘wasn’t able to’ -OR- ‘couldn’t’ are both correct.


Must / Have To - Mustn’t / Don’t Have to
Below are examples of must / have to / mustn’t / not have to

Examples
a. We have to get up early.
b. She had to work hard yesterday.
c. They will have to arrive early.
d. Does he have to go?

Usage
Use ‘have to’ in the past, present and future to express responsibility or necessity.
✦ ‘Have to’ is conjugated as a regular verb and therefore requires an auxiliary verb in the question form or negative.

Examples
a. I must finish this work before I leave.
b. Must you work so hard?

Usage
✦ Use ‘must’ to express something that you or a person feels is necessary.
✦ This form is used only in the present and future.

Examples
a. You don’t have to arrive before 8.
b. They didn’t have to work so hard.

Usage
The negative form of ‘have to’ expresses the idea that something is not required. It is however, possible if so desired.

Examples
a. She mustn’t use such horrible language.
b. Tom. You mustn’t play with fire.

Usage
The negative form of ‘must’ expresses the idea that something is prohibited - this form is very different in meaning than the negative of ‘have to’!

Examples
a. Did he have to leave so early?
b. He had to stay overnight in Dallas.

Usage
✦ The past form of ‘have to’ and ‘must’ is ‘had to’. Must does not exist in the past.


More on modal forms
Below is a general outline of modal forms and their possible time reference.

Examples of MODAL Simple Verb forms
a. He can answer your question. [Time reference: Present]
b. They should leave early tomorrow. [Time reference: Future]
c. He had to leave at 7 this morning. [Time reference: Past]

Example of MODAL Continuous form
a. He might be joking! [Time reference: Present]

Examples of MODAL Verbs Present Perfect
a. She can’t have been serious! [Time reference: Past]
b. They might have gone to the country. [Time reference: Past]

Example of MODAL Present Perfect Continuous Verb
a. They might have been working at that time. [Time reference: Past]

Modal Verbs of Probability
Below are examples of modal verbs of probability, which are used to express an opinion of the speaker based on information that the speaker has.

Example: He must be at work, it’s 10 o’clock.
In the above case, the speaker is 100% sure that the person is at work based on the speaker’s knowledge that the person in question usually works at during the day.

Examples
a. They must be in Spain by now.
b. She must have done well on the test.

Usage
Use ‘must’ plus the verb when you are 100% (or almost 100%) sure that something is the case.

Examples
a. She might come this evening.
b. David may invite Jesica to the match.
c. Jack might have gone to France.

Usage
Use ‘might’ or ‘may’ to express an opinion that you think has a good possibility of being true.

Examples
a. Jane could be at work.
b. Peter could have arrived late.

Usage
Use ‘could’ to express a possibility (one of many). This form is not as strong as ‘might’ or ‘may’. It is just one of a number of possibilities.

Examples
a. You can’t be serious!
b. They can’t have worked until late.

Usage
Use ‘can’t’ to express an opinion that you are 100% sure is NOT true.
✦ Notice that the past form remains ‘can’t have done’


STRUCTURE
In the present: Subject Modal Base Form Objects
I - must be/ might be/ could be/ can’t be - at work.
You - must be/ might be/ could be/ can’t be - at work.
He, She - must be/ might be/ could be/ can’t be - at work.
We - must be/ might be/ could be/ can’t be - at work.
They - must be/ might be/ could be/ can’t be - at work.

In the past: Subject Modal Perfect Form Objects
I - must have left/ might have left/ could have left/ can’t have left - early.
You - must have left/ might have left/ could have left/ can’t have left - early.
He, She - must have left/ might have left/ could have left/ can’t have left - early.
We - must have left/ might have left/ could have left/ can’t have left - early.
They - must have left/ might have left/ could have left/ can’t have left - early.