i dnt know the use of could .i still dnt understand

Could (past form of can) Describes an ability that someone had in the past: I could swim when I was young. You could see the boat sinking. They could tell he was nervous. Often used in auxiliary functions to express permission politely: Could I take this jacket with me? You could borrow my umbrella. Could you please let me pass you? Could I get you more water? Used to express possibility: All of them could ride in the van. You could always stay at our house. Could it be true? This plan could really work out.

thankyou so much

I want to continue it all in order to make strong my English