This is a tricky topic in English. It seems very simple that's why it’s misleading.
We think we already know it, but in fact we don’t. And when the time comes for us to use these words, we can’t.
But to learn the difference between “Can” and “Could,” the first thing you have to do is to change your wrong idea about “Could.”
First, let me ask you this, do you think “Could” is the Past Tense of “Can”?
If you answered “Yes,” you are right.
But remember: “Could” is not always the Past Tense of “Can.”
It’s common among English learners to think that “Could” is only the Past Tense of “Can.” No more, no less.
This isn’t true at all.
For example, what do you think about this question:
Could I borrow your pen?
Is it in the Past Tense?
Of course not. When you ask a question like this, you're asking your friend now – not yesterday or last week – if you can use the pen. It means today, right now and not any other time.
In fact, many examples and situations of “Could” are like this. This means, most of the time “Could” is in fact in the Present Tense, not Past Tense.
This is very different from the idea of many learners.
From now on, please remember that “Could” is an independent Modal Verb just like Can, Will, Have to, May, Might, Would, Should etc. So, it has both Present and Past Tense.
As you can see on the table above, “Could” is originally Present Tense and its Past Tense form is “Could Have.”
NOW that you’ve changed your wrong idea of “Could,” let’s start to compare it with “Can.”
What do you think about these questions?
Can I use your bathroom? - vs. - Could I use your bathroom?
Can I leave early? -vs. - Could I leave early?
What’s the difference between "Can" and "Could" ?
Well, both questions are used to ask permission. But as you might feel, “Can” suggests more familiarity and casualness. This means the speaker of the question has a closer relationship to the listener and uses a casual style of speaking.
On the other hand, “Could” suggests less familiarity but more formality and politeness. Maybe the speaker isn’t very close to the listener.
However, in the end, it all depends on the person. Coz a person might be familiar with someone but still polite in style of speaking, while a person might be unfamiliar with someone but still casual in speaking style.
NEXT, what do you think about these examples?
1. A: What are we gonna do this weekend?
B: We can go to the cinema. – vs. –
B: We could go hiking.
2. A: I’m travelling to Tokyo tomorrow. But I still don’t have a place to stay.
B: You can stay at a hostel. – vs. -
B: You could stay at my house.
How do you feel about these examples?
“Can” suggests higher possibility. This means “going to the cinema” and “staying at a hostel” are more possible to do or to happen, so the suggestion is stronger.
On the other hand, “going hiking” and “staying at my house” have less possibility. Maybe the speaker doesn’t usually go hiking or the house where he lives isn’t his own house. Perhaps he is living with his parents. This is why his suggestion is less possible to happen and weaker.
LASTLY, you can also use “Could” to talk about unrealistic or imaginary situations.
Ex. I’m so tired I could sleep here at the station.
I‘m so happy with him I could die.
Ohhh, I really hate him. I could kill him.
This is wonderful. I could stay here forever!
But in these situations, we can never use “Can.”
Hope you learned something!