用 can + 动词原形，来说你现在或平时能做的事。用 could 来说你过去能做的事。can 或 could 后面用动词原形，不加 to，也不加 -s。
can / could：说你能做什么
I can swim.
She can speak three languages.
When I was five, I could read.
He couldn't ride a bike last year.
Not "She cans swim. / I can to swim." — say "She can swim. / I can swim.". can 后面用动词原形：不加 -s，也不加 to。
试一试: Maria is a great swimmer. She ___ swim very fast. (can / cans / can to) 答案: can — 说你能做的事，用 can + 动词原形：She can swim。can 后面不加 -s（不能说 "cans"），也不加 to（不能说 "can to"）。
试一试: Last year Tom took lessons. Before that, he ___ ride a bike at all. (couldn't / can't / doesn't) 答案: couldn't — 说过去的能力，用 could / couldn't：he couldn't ride a bike。"Can't" 说的是现在，"doesn't" 是说现在的习惯，不是过去能不能。
Can I ...? 问可不可以
你已经知道 can 可以表示能做某事。Can I + 动词原形还可以用来问某件事可不可以做。
Can I use your phone?
Can I sit here?
Can we leave early today?
试一试: ___ I sit here, or is this seat taken? (Can / Do / Am) 答案: Can — 问某件事可不可以做，用 Can 开头：Can I sit here? "Do I sit" 问的是习惯性的事，"Am I sit" 不是正确的说法。
Could I ...? 更委婉的说法
Could I + 动词原形和 Can I 意思一样，但听起来更委婉更礼貌。跟不太熟的人说话，或者想格外客气的时候，就用它。
Could I borrow your pen, please?
Could I ask you a question?
Could we see the menu, please?
试一试: At the bank, Amina asks politely: "___ I speak to the manager, please?" (Could / Do / Should) 答案: Could — Could I 是问某事可不可以做的委婉礼貌说法：Could I speak to the manager? "Do I speak" 问的是习惯性的事，"Should I" 问的是建议，不是许可。
Could you ...? 请别人帮忙
要礼貌地请别人做某事，用 Could you + 动词原形，后面不加 to。
Could you help me, please?
Could you open the window?
Could you say that again?
Not "Could you to help me?" — say "Could you help me?". could you 后面不加 to：直接跟动词原形。
试一试: ___ help me carry these bags, please? (Could you / Could you to / Are you) 答案: Could you — 礼貌地请别人帮忙，用 Could you + 动词原形：Could you help me? 后面不加 to（不是 "Could you to help"），"Are you" 问的是对方正在做什么，不是请求帮忙。
试一试: You ask two polite favours: "___ I borrow your pen, please? And then ___ you check my spelling for me?" (Could … could / Could … do / Do … could) 答案: Could … could — Could 两种情况都能用：问自己能不能做某事，也可以请别人帮忙做某事。