Can is for ability now and casual requests: I can swim. Can you help me? Could is for general ability in the past, polite requests and possibility: I could swim at five. Could you help me? It could rain.
Same situation, two choices. Pick an example.
I can speak Spanish.
Ability now, and casual requests and permission.
I could speak Spanish as a child.
General ability in the past, and polite requests.
| Rule | Can | Could |
|---|---|---|
| Ability | Now: I can drive. | In the past, in general: I could drive at 17. |
| Requests | Casual: Can you…? | Polite: Could you…? |
| Possibility | Generally true: It can get cold here in winter. | One situation: It could rain later. |
| One success in the past | — | Not could: use was able to |
My grandfather _____ fix anything when he was young.
Paste an email or an essay. Clarigram marks the mistakes you actually make, shows the rule behind each one as a picture, and builds your practice from them.
Only for general ability. Could you…? is about now (a polite request), and It could rain is about the future (a possibility).