说过去能够做某事，用 could + 动词原形。could 后面不加 to：I could swim，不是 I could to swim。
could + 动词原形
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.". could 和动词之间不加任何词，不加 to。
试一试: When Amina was five, she ___. (could swim / could to swim / could swimming) 答案: could swim — could 后面直接用动词原形：could swim。could 后面不加 "to"，也不加 "-ing"。
人人都一样（不加 -s）
could 永远不变。he、she、it 也用 could，它后面的动词也不加 -s。
I could speak English.
She could speak English.
They could speak English.
Not "She could speaks English." — say "She could speak English.". 就算是 she，动词也保持原形，永远不是 could speaks。
试一试: When she was young, Sofia ___ French. (could speak / could speaks / coulds speak) 答案: could speak — Could 永远不加 -s，它后面的动词也保持原形：could speak（不是 "could speaks" 或 "coulds"）。
否定式：couldn't
说过去不能做某事，用 couldn't + 动词原形。（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.". couldn't 后面动词用原形，find，不是 found。
试一试: Hiroshi ___ his keys this morning. (couldn't find / couldn't found / didn't could find) 答案: couldn't find — 否定式是 couldn't + 动词原形：couldn't find（不是 "couldn't found"，也不是 "didn't could"）。
疑问式：Could 放最前面
要提问，就把 Could 放在主语前面，后面接动词原形。
Could you swim at five?
Could she speak French?
What could you do?
Not "Did you could swim?" — say "Could you swim?". 问句里 Could 放最前面，不需要 'did'。
试一试: ___ you ride a bike when you were six? (Could / Did could / Were) 答案: Could — 用 Could 开头，放在主语前面：Could you ride a bike? could 前面别加 "did"。
试一试: Remember what follows could, As a child, Lani ___ a bike. (could ride / could rides / could to ride) 答案: could ride — could 后面永远用动词原形：could ride。（每次都不加 -s，不加 "to"，不加 "-ing"。）