can 和 could：超越时态的差异
Can you help turn on the lights? 和 Could you help turn on the lights? 只是现在和过去的差别吗？
这边文章带你分辨can和could三大差别：确定度、能力描述和礼貌委婉度差别。让你不再混淆用法。更带你一次看懂英文不同的委婉、有礼貌语气，以后都不会不小心得罪人啦。
can和could的三大差别
「确定度」差别：
简单来说，用could叙述某件事情时，我们通常比用can时来得「不确定」
could：表达某件事「有可能属实」但「不确定」
They could be in the office now. (他们现在有可能在办公室。)
He could get fired soon. (他有可能最近会被开除。)
can：对某件事「有可能发生」的一般叙述
It can be difficult for a 3-year-old to learn to write. (通常3岁小孩要学写字会蛮难的。) You can easily get lost in the Taipei Train Station. (台北车站根本迷宫，很常有人迷路。)
can’t / cannot：表达某事「不可能」发生
That can’t be true. (不可能啦)
The lady cannot be over 60. (那位小姐不可能超过60岁。可以是表达难以置信。)
进阶用法：
Hurry up! They could have been waiting for a long time! (快点啦，他们有可能等很久了)
She could have been kidnapped. (她有可能被绑架了)
could have + p.p.：对过去做出猜测。
Hurry up! They could have been waiting for a long time! (快点啦，他们有可能等很久了)
She could have been kidnapped. (她有可能被绑架了)
could：对「过去」的一般叙述。 (这边没有猜测意味，因为过去的事情已经过了所以不用猜了）
It could be very hot in the summer here. (这里夏天以前有时候蛮热的) You could die from a cold quite easily in the 50s. (在50年代小感冒就蛮有可能致命了)
「能够；能力」叙述时的差别：
简单来说，我们用can叙述「一般能力、现在或未来能够做到的事」；用could表达「过去的能力」
can：表达「一般能力」
I can speak 5 languages (我会说五种语言。)
Tony can make a million dollar in just a few days. (Tony在仅仅几天内就能赚进一百万元。)
She can’t dance. (她不会跳舞)
can：表达「现在或未来能够做到的事」
I can’t see you. It’s too dark in here. (我看不到你，这里太暗了。)
I can go to the party tomorrow. (我明天能参加派对。)
could ：表达「过去能力」
I could do backflips when I was 22. (我22岁的时候会后空翻。)
Amy couldn’t sing vert well when she was little. (Amy小时候还不太会唱歌。)
进阶用法：
could have + p.p.：表达「过去有能力或机会做某事但却没有做」
You could have told me the truth, but you chose not to.（你大可跟我说实话，但你选择说谎)
I could have punched you in the face. But I love you. (我大可揍你一拳，但我爱你。)
「礼貌、委婉度」差别：
简单来说，通常用could做出请求或是提出建议，会比用can来得委婉、有礼貌。
can：做一般请求(较没有礼貌)
Can you take my order? (可以帮我点餐了吗？)(这样问服务员蛮没礼貌喔，感觉不耐烦。)
Can you yield your seat? (可以让位吗？)
could：做较有礼貌的请求。
Could you turn down the volume a little bit? Thank you. (音乐可以麻烦小声些吗？谢谢。)
Could I use the restroom? (我能借个厕所吗？)
could：有礼貌、委婉地提出建议、想法
We could take five if you’d like to. (如果你想要的话，我们可以休息五分钟。)
You could make the goals a bit more achievable. (你可以把目标订得可实现一点。)
通常英文会用「距离感」来营造委婉、有礼貌的语气。常见的用法有「过去式代替现在式」、「加入不确定性」、以及「藏起主词」。
过去式代替现在式
这应该是最常见的用法了，可以直接看例句。
Can you pass me the pen? → 礼貌用法升级 → Could you pass me the pen?
Will you clean your room? → 礼貌用法升级 → Would you clean your room?
加入不确定性
在做请求时加入不确定性，可以让请求显得不那么武断。
Could you pass me the pen? → 礼貌用法升级 → I was wondering if you could pass me the pen?
Are you free tonight → 礼貌用法升级 → Are you by any chance free tonight?
Can I take a day off? → 礼貌用法升级 → Would it be okay if I take a day off?
Could I use your laptop? → 礼貌用法升级 → Would you mind if I use your laptop?
藏起主词
直接用You开头常常会有命令人的感觉，这时候可以改用被动态来藏起主词。
You should sign the contract by tomorrow. (像命令)
The contract should be signed by tomorrow. (主词藏起来委婉、礼貌多了)
Everyone should park their cars on the 3rd Street. (像命令)
All carrs should be parked on the 3rd Street. (主词藏起来委婉、礼貌多了)
