Use of Could 

The word "could" is a modal verb, and it's used in several ways in English.

Main uses:

1. Past ability

To talk about something someone was able to do in the past.

Example:

When I was younger, I could run 10 kilometers without stopping.

She could play the piano at the age of five.

2. Possibility (present or future)

To express that something is possible, but not certain.

Example:

It could rain later, so take an umbrella.

This plan could save a lot of money

3. Polite requests

To ask for something in a more polite or less direct way.

Example:

Could you help me with this homework?

Could I borrow your pen, please?

. Suggestions

To politely suggest something.

Example:

You could try restarting your computer.

We could go to the beach this weekend.

5. Unreal or hypothetical situations

To talk about imagined or unlikely situations.

Example:

If I had more free time, I could learn another language.

This could be the start of something amazing.

6. Past possibility

To say that something was possible in the past but didn't happen.

Example:

I could have called you, but I forgot.

She could have been a great singer if she had trained more.