"could" is usually used as the past form of "can". However, it can be used when you want to express (with no past tense meaning) :
1. The polite requests:
e.g: "Could you please ..." is more polite than "can you please.."
2. The possibilities in the present or future with a little certainty or doubtfulness:
e.g: "It could be about six o'clock" is used when you give out your guess on the present time.
"What are you going to wear to the party?
- I don't know. I could wear jeans, I guess"
E.g: "Even if he had been there, he couldn't have helped you"
4. The indirect sentences.
E.g: He said he couldn't arrive before six o'clock
5. The conditional sentences.
"I could come earlier, if necessary"
"If you tried, you could do that work"
I hope this can help you.
Student or Learner