Can, Could, Be Able To

An auxiliary verb is used with the main verb for helping in order to express the tense, mood, or voice of the main verb. ‘Can’ and ‘could’ are called modal auxiliary verbs. They are used to show the possibility or ability to do something in sentences. The use of ‘can and could’ differ in the tenses. ‘Be able to’ use the verb as the main verb and it is not an auxiliary verb. ‘Can’ and ‘could’, ‘be able to’ are used in a sentence for possibility and ability.

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Can

‘Can’ is an auxiliary verb also a kind of modal auxiliary verb. ‘Can’: is used to show about possibility and ability, make requests and ask for or give permission. This verb is used in only one form i.e. can hence it is invariable. 

Structure of Can

The following is the basic structure for can that is used in sentences.

Subject  + (can) auxiliary verb + main verb

Use of Can

The uses of ‘can’ can be better understood with the help of examples. The uses of ‘can’ are given below.

• ‘Can’ is used to show the possibility of something like what we are able or free to do. For examples:

1. She can cook dinner. 

2. Ali can speak Urdu.

3. I cannot teach you. 

4. Can you see me?

• Generally, we use ‘can’ for the present. But it is also used when we make present decisions about future ability. For example:

1. Can you play badminton with me? (present)

2. I can help you tomorrow. (future)

• ‘Can’ is also used for requests and orders. When we use ‘can’ in a question to ask someone to do something. It is not a kind of real question because we do not really want to know if the person is able to do that work but we want them to do it. This way of using ‘can’ is informal. For example:

1. Can you stand up for a while, please?

2. Can you turn the alarm off?

3. Can you come here?

4. Can you keep quiet?

• ‘Can’ is also used to ask or give permission in a sentence. For example:

1. Can I eat this cookie?

2. You can have the seat.

Could

‘Could’ is also a modal auxiliary verb. It is used to talk about the possibility or ability of the past, to make requests. The main verbs in the sentences are always the bare infinitive. It is used in only one form that is, could, hence it is invariable.

Structure of Could

The following is the basic structure of could that is followed in sentences.

subject + (could) auxiliary verb + main verb

We can understand the structure of could with the following example.

Use of Could

The auxiliary verb could is used to show the possibility or ability of the past. 

• ‘Could’ is used to talk about some possibility of the past about the freedom and ability to do something. For example:

1. I could sing properly when I was 12 years old.

2. Rohan could speak different languages.

3. When we arrived at the restaurant, we could not open the door. 

4. Could you see what he was doing?

• This auxiliary verb ‘could’ is also used for requests. Often ‘could’ is used in a form of questions to ask to do something. For example:

1. Could you tell me the direction where the hotel is, please?

2. Could you give me a book, please?

Be Able To

‘Be able to’ is not a modal verb. It is simply an adjective addition to a verb. Here the verb is an adjective followed by the infinitive. Sometimes ‘be able to’ is used instead of  ‘can’ and ‘could’. ‘Be able to’ is used to show ability about something in a sentence. ‘Be able to’ can be used in all tenses. For example:

1. I was able to run.

2. I will be able to run.

3. I have been able to run.

‘be able to’ has an infinitive form. For example: 

1. I would like to be able to play football better.

Structure of ‘Be Able To’

The basic structure for ‘be able to’ is as follows.

subject + be + able + infinitive (to)

Use of ‘Be Able To’

Following are the uses of ‘be able to’ with examples.

• ‘Be able to’ is used for showing ability in a sentence. We use it to express ability. The word ‘Able’ has an adjective meaning i.e. to have the power, ability, skill or means to do something. Hence ‘be able to’ can be used instead of ‘can’ or ‘could’ for ability.

• The use of ‘be able to’ is possible in all tenses but we know that ‘can’ is used only in the present and ‘could’ is in the past for ability.  ‘Can’ and ‘could’  do not have an infinitive form. Hence ‘be able to’ is used when other tenses or infinitives are used. Look at these examples:

1. I have been able to dance since I was ten. (present perfect)

2. Dinesh will be able to walk properly very soon. (future simple)

3. Komal would like to be able to drive a car. (infinitive)

Conclusion

So we have learnt about the modal auxiliary verbs and the use of ‘can and could’ with examples but ‘be able to’ is not an auxiliary verb. But it is convenient to learn about it here because it is often used as ‘can’ and ‘could’. We often use these in our normal sentences so we must be aware of their correct use.