Let’s look at how we use can and could modal verbs in more detail now.
When can and could have different uses
Present ability(Can) vs Past Ability(Could)
Ability is one of the most common uses of Can and Could. The main difference is that we use can in the present and could for the past.
I can play the piano now.
I could play the piano when I was a child.
I can drive a car.
I could drive well but now I need more practice.
We use can(or will be able) for the future tense.
I can go to the party next week.
If you need to express ability in a perfect tense like the present perfect then it is better to use “be able to”
I have been able to fix all the tiles on the roof.
I haven’t been able to change the light bulb.
Realistic Possibility(Can) vs Unrealistic Possibility(Could)
When we use can, we are realistic and it is true.
I can stay with my friend for a few days.
When we use could we are being unrealistic.
I could stay with my friend for one year.
It is important to know the context to understand this better and you need to understand more about the speaker’s opinions.
For example, imagine that you need to borrow money from a friend called John but he is not very generous.
We could borrow money from John.
In this case, you are being sarcastic because John is not generous(stingy)
More examples of unrealistic could:
I could eat a horse!
I feel great, I could run a marathon!
I could learn English in one week!
To offer something (Can)
We use can to offer to do something.
Can I help you with the washing up?
Can I help you with your homework?
To suggest something (Could)
We use could when we want to suggest something. A good example is when we suggest something to do.
We could go to the football game tonight.
We could use a hammer to fix the wall.
When you use can in this situation, it means that you are allowed to do that activity.
We can stay out until 11 pm.(our parents allow us)
We can go to the football game. (some authority allows us)
When can and could have the same uses
To request something (Can or Could)
We can use both Can or Could to request something.
Could is more formal than Can
Can I have the salt, please?
Could you pass me the salt, please?
For this reason in some situations, it is more common to use can(informal) and in other situations, it is more common to use could(formal, polite).
Can I bring my friend with me?
Could you open a window?
To ask permission(Can or Could)
The same small difference applies to asking for permission. Could is formal and polite and can is more informal.
Can I use the bathroom?
Could I use the bathroom?
Can I borrow your lawnmower?(a friend)
Could I borrow your lawnmower? ( an acquaintance)
More uses of Can and Could
Could is the conditional form of can
If you want to say “would can” then you need to use could. This is used for the second and third conditional.
If I had a lot of money, I could buy I new car.
If I had left early, I could have watched the Rugby game.
Can and could in reported speech
We also use could in place of can for reported speech:
I can go to the cinema later.
He said that he could go to the cinema later.