Can means be able to, be allowed to, or be possible now.

Could can mean was able to, might be possible, or would be possible under certain conditions. It is also often used to sound more polite.

In other words:

can = present ability / permission / possibility
could = past ability / polite request / hypothetical possibility
can = direct
could = softer or less certain

✅ I can speak Spanish.

Here, can means present ability. The person is able to speak Spanish now.

✅ When I was younger, I could run faster.

Here, could means past ability. The person was able to run faster before.

✅ Can you help me?
✅ Could you help me?

Both are correct, but could sounds more polite or softer.
Can you help me? is direct.
Could you help me? sounds a little more courteous.

✅ It can get cold here in winter.
✅ It could rain later.

Can describes something that generally happens or is possible.
Could suggests a possibility that is less certain.

The common mistake is thinking could is only the past tense of can.

It can be, but not always.

Could also works for:

Examples:

✅ Could I ask you a question?
✅ We could try a different approach.
✅ If I had more time, I could finish it.

Another mistake is using can when the situation needs a softer tone.

❌ Can you send me the report now?
This is grammatically correct, but it may sound too direct.

✅ Could you send me the report when you have a chance?
This sounds more polite and professional.

Use this rule:

Can is for what is possible, allowed, or true now.
Could is for the past, politeness, uncertainty, or imagination.

Formula:

can = ability + permission + now
could = past + polite + possible