can / could
表示能力、请求或可能性，could 更委婉。
can / could + 动词原形
要点
- can 表示能力（会、能）或可能性。
- could 是 can 的过去式，也用于更委婉的请求。
- 后面一律接动词原形，不受主语人称和数的影响。
- 否定 can't / couldn't，疑问把 can / could 提到句首。
- 说过去某一次真的办成了，不用 could 而用 was / were able to：They were able to get home before dark.（那天赶回来了）—— could 只说当时具备能力，不管事成没成。
- can 只有 can / could 两张脸，将来与完成都得借 be able to：She will be able to drive next year. ／ He has been able to walk again. —— 写 will can、is caning 都是造出来的形式。
- 提请求的三档客气度：Can I …?（熟人）／ Could I …?（客气）／ May I …?（正式）—— 只差礼貌，回答时常常仍用 can：Yes, you can.
例句
Can you swim?
你会游泳吗？
I can see a dog.
我能看到一只狗。
Could I have some water?
我能要点水吗？
Could you do me a favor?
你能帮我一个忙吗？
After the operation, he was able to walk without help.
手术后他不用人扶就能走路了。
I can't help laughing at his jokes.
他的笑话让我忍不住想笑。
易错提示
- can 后面直接接动词原形，中间不加 to。
- 请求句里的 could 不是过去时，只是更客气。
- 情态动词不加 -s、不带 to、不接 -ing：he cans、to can、can going 一个都不成立 —— 这条同样管着 may / must / should。
- can't help doing 是整块的说法（忍不住做）：I can't help laughing. —— help 后面接 -ing，不接 to do。