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Modal Verbs Can and Could
'Can' and 'could' are both modal verbs that are mainly used to express ability, possibility, permission, and requests. 'Could' is used to express past ability, possibility, or permission. It can also be used to make polite requests.
Can and Could: Functions
'Can' and 'could' are used to:
Talking about Ability
One of the most common uses of the modals 'can' and 'could' is talking about abilities.
Can
Can is used to talk about abilities that someone has in the present or future, based on their knowledge or talent. For example:
I can drive.
She can sing really well.
Could
Could is used to talk about general abilities in the past. We use it to talk about what someone was able to do in the past.
My uncle could speak five languages.
Use of 'could' indicates that his uncle can no longer speak five languages, or maybe he is no longer alive.
By the time he was seven, he could read and write.
Be Able to Vs. Can
'Can' and 'be able to' generally have the same meaning when used to talk about abilities, and in most situations either one can be used. However, 'be able to' is considered more formal than 'can', and may be more appropriate in formal or professional contexts.
I can play the piano. = I am able to play the piano.
I could jump very high when I was younger. = I was able to jump very high when I was younger.
Additionally, 'can' and 'could' cannot be used in every verb tense and form, because they are modals and do not conjugate. When using perfect tenses or gerunds to talk about abilities, it is more appropriate to use 'be able to.'
I have been able to dance since I was three years old.
Here, we cannot use 'can' or 'could' and still convey the same meaning.
I like being able to watch my favorite TV shows on YouTube.
Verbs such as 'like' take a gerund as complement. So we cannot use 'can' or 'could' after them.
On the other hand, 'can' and 'could' are used with verbs such as see, hear, understand, notice, etc. that denote an involuntary action. 'Be able to' is not typically used with these verbs.
I can hear a noise.
Do not say "I am able to hear a noise."
I could understand every word he said.
Do not say "I was able to understand every word he said."
Be able to or Could?
Both 'could' and 'be able to' can be used to talk about abilities in the past. But they are not always interchangeable.
For example, if you want to talk about a specific achievement, you must use 'be able to' and using 'could' would create ambiguity. Look at the examples below:
I was able to pass the driving test on the first try.
In this example, we are talking about a specific achievement in the past and we cannot use 'could'.
I was able to reserve a table, although it was very busy.
However, this distinction is only relevant in affirmative sentences. In negative sentences both 'could' and 'be able to' are acceptable.
I couldn't reserve a table.
I couldn't pass the driving test on the first try.
Warning!
Two modal verbs cannot be used in one sentence at the same time, i.e. you cannot use 'can' or 'could' with other modals like 'should' or 'would.' In such cases, 'be able to' is used instead of can/could.
You should be able to choose your own religion. (Not You should can choose ...)
I would be able to give you an appointment for January 3rd. (Not I would can give you ...)
Talking about Possibility
'Can' is commonly used to talk about something that is possible in the present or future, while 'could' is commonly used to talk about possibilities in the past.
Can you come back tomorrow?
Here, the sentence means 'Is it possible for you to come back tomorrow.' So, 'can' does not show an ability, but a possibility.
The auditorium can be emptied in five minutes.
'Could' is also used to talked about a possibility that did not actually happen. Keep in mind that 'can' is not used in this case.
You could have been killed!
I could have become the greatest player in the world.
Making a Request
'Can' is also used to make a request. But keep in mind that this use of 'can' is very informal.
Can I borrow your book?
Can you help me with my homework?
When making a request, 'could' does not necessarily refer to the past tense. It is used in the present tense to make a polite request.
Could I have a glass of water, please?
Here, the request is being made politely at the present time.
Asking for or Giving Permission
'Can' is also frequently used to ask for permission to do something or to give somebody permission to do something.
You can take the car, if you want.
giving permission
Can we wear jeans at work?
asking for permission
'Could' is only used to ask for permission to do something and is more formal than 'can'. In this case too, it does not refer to a past time, but is used to make the sentence more polite. Keep in mind that asking for permission always takes the form of a yes/no question. Here are some examples:
Could I use your phone?
Could I leave?
Can't or Cannot?
The words 'can' and 'not' can be combined into one word, 'cannot', which is more formal and commonly used in written English. In informal English, it is more common to use the contraction 'can't' instead of 'cannot.'
He cannot believe what she's saying right now.
Clause Types that Require 'Could'
Some specific types of clauses call for the use of past modals, including 'could':
Reported Speech
Reported speech involves the process of backshift which requires every verb in the reported clause to shift one tense back. Accordingly, if the direct speech includes the modal 'can,' the reported speech must use 'could' in its place. Here are some examples:
She asked if I could help her.
She said I could leave early if I finished my work.
Past Subjunctives
The verb 'wish' and some other expressions like 'as if,' 'if only,' etc. require the clause that follows them to be in the past subjunctive mood. The past subjunctive uses the past simple tense of the verbs, which means 'could' is always used in these clauses instead of 'can.' Take a look at the examples below:
I wish I could speak Japanese.
If only I could turn back time.
Review
'Can' is used a lot in daily English and even in formal writing. The table below summarizes the different functions it fulfils:
to talk about abilities | She can speak Spanish. |
|---|---|
to request something informally | Can you give me a pen? |
to talk about possibilities | You can either take the bus or walk to work. |
to give or ask for permission | Can I put my clothes on your bed? |
'Could' is the past tense of 'can,' but it is not always talking about the past. Look at the table below:
to talk about past abilities | I could swim back then. |
|---|---|
to make a polite request | Could you please close the window? |
to talk about possibilities in past | It could be dangerous to be out there at night. |
to ask for permission | Could I leave a bit early today? |
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