Expressing Ability, Possibility and Obligation

In this lesson, you'll learn how to express ability, possibility, and obligation. In English ability, possibility, and obligation are often expressed using a type of verb called a model verb. These verbs are also discussed in this lesson.

Plenty of examples are included to help you learn how to express possibilities, abilities, and obligations in the English language.

Introduction

In this lesson you'll learn how to use modal verbs in English, as well as other ways to express ability, possibility, permission, and obligation.

Modal Verbs

Modal verbs are a special class of verbs that add degrees of ability, possibility, permission or obligation to another verb. The common modal verbs in English are:

These verbs are very common in English, so it is worth spending time to learn how to use them properly.

Modal verbs have several properties that make them different from other verbs in English, such as:

They are only used in the present tense, except can and will, which also have a simple past tense.
Except for must, they have an alternate form which adds a sense of uncertainty to the meaning.
They do not add an s in the third-person.
They do not have an infinitive form.
The verb do is not used with modal verbs when asking questions or in negative statements.

The following sections will discuss each of the modal verbs.

Can

The most common way to express an ability to do something is with the verb can. Can is a modal verb and is only used in the present and simple past tense. Can is also used to indicate possibility and permission.

Could is another form of can. It has two meanings:

It is the past tense of can.
It is used to show that something is able to happen in the future, but whether it will happen is uncertain.

Could have means that an action was possible in the past, or there was the ability to do something in the past, but it wasn't done. Could have is often shortened to could've.

For other tenses than present and past, be able is used. The following table shows the the different forms.

Infinitive	to be able
Present	can
Past	could
Present Perfect	have been able
Past Perfect	had been able
Conditional	would be able
Future	will be able

Examples

She can swim across the lake.	She has the ability (and permission) to swim across the lake. Notice that no s is added to can like most other verbs. Also note that there is no to before swim.
Can she swim across the lake?	Does she have the ability (or permission) to swim across the lake? Note that the verb do is not used when asking a question.
She cannot swim across the lake.	Note that the verb do is not used and can and not are combined into a single word.
She could swim across the lake (if she wanted to).	She has the ability to swim across the lake if she wants to.
She could swim across the lake (before she broke her leg)	The past tense of can. She had the ability to swim across the lake in the past, but she does not have the ability now.
She will be able to swim across the lake.	She has the ability to swim across the lake if she tries in the future.
She could've swum across the lake.	She had the ability to swim across the lake, but did not.
Could she have swum across the lake?	Did she have the ability to swim across the lake at some point in the past
She could not have swum across the lake.	She did not have the ability to swim across the lake in the past.

May

The meaning of may is similar to that of can. It is used for asking permission, or expressing possibility. Be allowed is also used to indicate permission.

Might is another form of may. It is used to indicate uncertainty. It is also used to make very polite requests.

May have and might have are both used to indicate that an event possibly happened. Might have is often shortened to might've.

Infinitive	to be allowed
Present	may
Past	was allowed
Present Perfect	have been allowed
Past Perfect	had been allowed
Conditional	would be allowed
Future	will be allowed

Examples

May I go to the store?	Polite way to ask permission.
May I have something to drink?	Polite way to make a request.
You may have one chocolate bar.	Give permission.
John may have an extra pen.	Express possibility.
I may go to the concert.	Express possibility.
I may not have time to eat dinner.	Express possibility that an action will not happen.
She may be able to help you.	Express possibility.
May all your dreams come true.	A formal way to express hope that something happens.
It might happen.	Express possibility.
It might have happened.	Express that something in the past possibly happened.
I may have lost the keys.	Express that something in the past possibly happened.
I might not be able to do it.	Express possibility that something will not happen.

Must

The modal verb must is used to express obligation, certainty and to give orders. Must can only be used in the present tense. The verb have to is also used to express obligation. The past tense of have to is had to. Must has a slightly stronger sense of obligation than have to.

Infinitive	to have to
Present	must (have to)
Past	had to
Present Perfect	have had to
Past Perfect	had had to
Conditional	would have to
Future	will have to

Examples

We must not forget to take out the garbage.
He must've forgotten about it.
She must pass the exam, or she will fail the course.
She has to pass the exam.
She had to pass the exam to be able to pass the course.
I'll have to do it eventually.

In the combination must have, we are indicating that we are certain that an action happened in the past. Must have is often shortened to must've.

I must have given you your laptop back.
They must have known what would happen if they did that.

In the combination must be, we are indicating that we are certain that an action is currently happening, or a specific state currently exists.

We must be going too slow
We must be almost at the lake by now.

Ought to is another way to indicate obligation. It is not as strong a sense of obligation as must or have to, and has the same meaning as should.

Shall

Shall has the following meanings:

to indicate a sense of obligation
to indicate the future tense
to make suggestions

The most common usage of shall is for making suggestions. Shall is often shortened to 'll.

Shall, along with will, is used to indicate the future tense, though will is much more common. Both shall and will are often shortened to 'll.

Should is another form of shall, and is used to indicate obligation or to make a suggestion.

Examples

Shall we go?	Making a suggestion.
What shall I do?	Asking for suggestions.
What'll I do?	Asking for suggestions.
He'll do it.	Future tense. Shortened form.
We shall never surrender!	Future tense.
You shall not pass!	Future tense.
You should get more exercise.	Expressing an opinion.
I should study.	Indicating obligation.
Should I study?	Asking for an opinion.
You should not study.	Giving an opinion.
You shouldn't study.	Giving an opinion. Shortened form.
We should go see the new movie.	Offering a suggestion
You should study more.	Giving an opinion or offering a suggestion.
Shouldn't you study more?	Making a suggestion.

Will

The future tense in English is formed by using the present tense of the verb will. Will is usually shortened to 'll in conversational English, when it follows the subject of a sentence. The negative form will not is often shortened to won't.

The conditional tense in English is formed using would, which was originally the past tense of will. It is used to indicate that an event will happen only if certain conditions are met, or won't happen because of certain conditions. It is also used to indicate general uncertainty about whether or not an event will happen. In conversational English would is often shortened to 'd when it follows the subject of a sentence.

Will you go shopping after work?
We will talk about it later.
We'll go if you really want us to.
We'll have time to eat dinner.
We won't have time to eat dinner.
Why won't we have time to eat dinner?
Why won't it work?
I'm sure it'll work.
I would go to the concert if I had more money.
I'd go to the concert if I had more money.
Would you really do that?
I wouldn't do that.
Why wouldn't they want to do that?

Asking Permission

There are several ways to ask permission in English. The best way to explain how they are used is with examples.

Can I go to the store?	This is a very common and informal way to ask permission.
Could I go to the store?	This is a slightly more polite way of asking permission.
May I go to the store?	This is a more formal or polite way of asking permission?
Am I allowed to go to the store?	This is a less common way of asking permission. It indicates a bit of uncertainty that the answer will be yes.
Is it OK if I go to the store?	This is a way to ask permission, while indicating you won't mind if the answer is no.
Is it all right if I go to the store?	This is a way to ask permission, while indicating you won't mind if the answer is no.

To indicate you have permission or don't have permission to do something, you can use the verb can or the phrase be allowed. For example,

I can go to the store.
I'm allowed to go to the store.
I can't go to the store.
I'm not allowed to go to the store.

To tell people that they do not have permission, you can use one of the following phrases:

No. (most common)
You may not. (Strongly indicate a person does not have permission)
Forget it! (Strongly indicate a person does not have permission)
You're not allowed. (Reminding a person they do not have permission)

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