Three Uses of Can
1. Indicates Ability “Can”
I can swim.
2. Indicates Possibility “May”
He can’t be at home.
3. Indicates Permission “May”
Can you open the door?
Three Uses of Could
1. Indicates Ability “Could”, the Past Tense of Can
I could swim when I was five.
2. Indicates Permission “Could”, More Polite than Can
Could you open the door?
3. Makes Suggestions
You could try this.