表示过去能做到的事，用 could 加动词原形。could 后面不加 to：I could swim，不说 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 和动词中间不加 to。
试一试: Yasmin: Was Mateo good at sports? Lani: Yes, he ___ soccer very well at ten. 补全 Lani 说的话。 (could play / could to play / could playing) 答案: could play — Yasmin 用 Was 来问，at ten 也已经过去了，所以这里说的是过去的能力。could 后面的动词用原形，不加 to，也不加 -ing。
所有人都一样（不加 -s）
could 永远不变。He、she、it 也用 could，后面的动词不加 -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，绝不说 could speaks。
试一试: Ethan: Was Sofia good at languages? Camila: Yes, she ___ two languages at ten. 补全 Camila 说的话。 (could speak / could speaks / coulds speak) 答案: could speak — Could 永远不变，后面的动词就算是 she 也不加 -s。也没有 "coulds" 这种说法。
否定：couldn't
表示过去没能做到的事，用 couldn't 加动词原形。（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 后面的动词用原形，是 find，不是 found。
试一试: Camila: Why were you outside so long? Hiroshi: It was dark. I ___ the number on the door. 补全 Hiroshi 说的话。 (couldn't see / couldn't saw / don't see) 答案: couldn't see — Hiroshi 说 It was dark，所以他说的是已经过去的时刻。couldn't 后面的动词用原形，是 see，不是 saw；don't see 说的是现在。
疑问句：Could 放最前
提问时，把 Could 放在人的前面，再接动词原形。
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 放在最前面，不需要 'did'。
试一试: Matilda: ___ you cook when you were young? Lars: Only eggs! 补全 Matilda 的问题。 (Could / Did could / Were) 答案: Could — When you were young 说明是过去，过去的问句要把 Could 放在人的前面，前面不用加 did。
could 后面接什么？
合起来记：could 后面的动词永远不变，不加 to，不加 -ing，不加 -s，也绝不用过去式。
She could cook at twelve.
We could walk to school.
They could read music.
Not "She could cooked at twelve." — say "She could cook at twelve.". could 后面的动词永远不变，不加 -ed，不加 -ing，也不加 -s。
试一试: Sofia: Were you a fast child? Lani: Very! I ___ all the way home when I was five. 补全 Lani 说的话。 (could run / could ran / could to run) 答案: could run — Sofia 用 Were 来问，Lani 又说了 when I was five，所以是过去。could 后面的动词永远不变：是 run，不是 ran，也不加 to。
Could you help me?（请求）
你已经知道 could 可以表示过去的能力。Could you...? 也可以用来礼貌地请别人现在帮你做点什么。
Could you help me, please?
Could you open the window?
Could you say that again?
试一试: Matilda: Could you take this bag? My hands are full. Ethan: ___ 选出 Ethan 会说的话。 (Of course. Here, give it to me. / Yes, I could. I was strong then. / No, I couldn't find it.) 答案: Of course. Here, give it to me. — Matilda 现在两只手都拿满了，所以她是在请人帮忙，不是在问过去的事。说以前能做到什么，或者说找东西的回答，都不合适。