Could / Couldn't (A1 Grammar)

Use could + the base verb for something you were able to do in the past. No to after could: I could swim, not I could to swim.

could + base verb

When Amina was five, she could swim.
Hiroshi could ride a bike at six.
I could read when I was four.
Not "She could to swim." — say "She could swim.". Nothing goes between could and the verb, no to.
Try it: When Amina was five, she ___. (could swim / could to swim / could swimming) Answer: could swim — After could, use the base verb on its own: could swim. No "to" and no "-ing" after could.

could never changes. He, she, it use could too, and the verb after it has no -s.

I could speak English.
She could speak English.
They could speak English.
Not "She could speaks English." — say "She could speak English.". Even with she, the verb stays plain, never could speaks.
Try it: When she was young, Sofia ___ French. (could speak / could speaks / coulds speak) Answer: could speak — Could never takes -s, and the verb after it stays plain too: could speak (not "could speaks" or "coulds").

For something you were not able to do, use couldn't + the base verb. (couldn't = could not.)

She couldn't find her keys.
I couldn't swim when I was three.
They couldn't come yesterday.
Not "She couldn't found her keys." — say "She couldn't find her keys.". After couldn't, the verb is plain, find, not found.
Try it: Hiroshi ___ his keys this morning. (couldn't find / couldn't found / didn't could find) Answer: couldn't find — The negative is couldn't + the base verb: couldn't find (not "couldn't found," and not "didn't could").

To ask, put Could before the person, then the base verb.

Could you swim at five?
Could she speak French?
What could you do?
Not "Did you could swim?" — say "Could you swim?". For a question, Could goes first, you don't need 'did'.
Try it: ___ you ride a bike when you were six? (Could / Did could / Were) Answer: Could — Start the question with Could, before the person: Could you ride a bike? You don't add "did" before could.
Try it: Remember what follows could, As a child, Lani ___ a bike. (could ride / could rides / could to ride) Answer: could ride — After could, always the base verb: could ride. (No -s, no "to", no "-ing", every time.)