can 和 could：超越時態的差異
Can you help turn on the lights? 和 Could you help turn on the lights? 只是現在和過去的差別嗎？
這邊文章帶你分辨can和could三大差別：確定度、能力描述和禮貌委婉度差別。讓你不再混淆用法。更帶你一次看懂英文不同的委婉、有禮貌語氣，以後都不會不小心得罪人啦。
can和could的三大差別
「確定度」差別：
簡單來說，用could敘述某件事情時，我們通常比用can時來得「不確定」
could：表達某件事「有可能屬實」但「不確定」
They could be in the office now. (他們現在有可能在辦公室。)
He could get fired soon. (他有可能最近會被開除。)
can：對某件事「有可能發生」的一般敘述
It can be difficult for a 3-year-old to learn to write. (通常3歲小孩要學寫字會蠻難的。) You can easily get lost in the Taipei Train Station. (台北車站根本迷宮，很常有人迷路。)
can’t / cannot：表達某事「不可能」發生
That can’t be true. (不可能啦)
The lady cannot be over 60. (那位小姐不可能超過60歲。可以是表達難以置信。)
進階用法：
Hurry up! They could have been waiting for a long time! (快點啦，他們有可能等很久了)
She could have been kidnapped. (她有可能被綁架了)
could have + p.p.：對過去做出猜測。
Hurry up! They could have been waiting for a long time! (快點啦，他們有可能等很久了)
She could have been kidnapped. (她有可能被綁架了)
could：對「過去」的一般敘述。(這邊沒有猜測意味，因為過去的事情已經過了所以不用猜了）
It could be very hot in the summer here. (這裡夏天以前有時候蠻熱的) You could die from a cold quite easily in the 50s. (在50年代小感冒就蠻有可能致命了)
「能夠；能力」敘述時的差別：
簡單來說，我們用can敘述「一般能力、現在或未來能夠做到的事」；用could表達「過去的能力」
can：表達「一般能力」
I can speak 5 languages (我會說五種語言。)
Tony can make a million dollar in just a few days. (Tony在僅僅幾天內就能賺進一百萬元。)
She can’t dance. (她不會跳舞)
can：表達「現在或未來能夠做到的事」
I can’t see you. It’s too dark in here. (我看不到你，這裡太暗了。)
I can go to the party tomorrow. (我明天能參加派對。)
could ：表達「過去能力」
I could do backflips when I was 22. (我22歲的時候會後空翻。)
Amy couldn’t sing vert well when she was little. (Amy小時候還不太會唱歌。)
進階用法：
could have + p.p.：表達「過去有能力或機會做某事但卻沒有做」
You could have told me the truth, but you chose not to.（你大可跟我說實話，但你選擇說謊)
I could have punched you in the face. But I love you. (我大可揍你一拳，但我愛你。)
「禮貌、委婉度」差別：
簡單來說，通常用could做出請求或是提出建議，會比用can來得委婉、有禮貌。
can：做一般請求(較沒有禮貌)
Can you take my order? (可以幫我點餐了嗎？)(這樣問服務員蠻沒禮貌喔，感覺不耐煩。)
Can you yield your seat? (可以讓位嗎？)
could：做較有禮貌的請求。
Could you turn down the volume a little bit? Thank you. (音樂可以麻煩小聲些嗎？謝謝。)
Could I use the restroom? (我能借個廁所嗎？)
could：有禮貌、委婉地提出建議、想法
We could take five if you’d like to. (如果你想要的話，我們可以休息五分鐘。)
You could make the goals a bit more achievable. (你可以把目標訂得可實現一點。)
Could用法整理影片版教學：
委婉、有禮貌語氣整理
通常英文會用「距離感」來營造委婉、有禮貌的語氣。常見的用法有「過去式代替現在式」、「加入不確定性」、以及「藏起主詞」。
過去式代替現在式
這應該是最常見的用法了，可以直接看例句。
Can you pass me the pen? → 禮貌用法升級 → Could you pass me the pen?
Will you clean your room? → 禮貌用法升級 → Would you clean your room?
加入不確定性
在做請求時加入不確定性，可以讓請求顯得不那麼武斷。
Could you pass me the pen? → 禮貌用法升級 → I was wondering if you could pass me the pen?
Are you free tonight → 禮貌用法升級 → Are you by any chance free tonight?
Can I take a day off? → 禮貌用法升級 → Would it be okay if I take a day off?
Could I use your laptop? → 禮貌用法升級 → Would you mind if I use your laptop?
藏起主詞
直接用You開頭常常會有命令人的感覺，這時候可以改用被動態來藏起主詞。
You should sign the contract by tomorrow. (像命令)
The contract should be signed by tomorrow. (主詞藏起來委婉、禮貌多了)
Everyone should park their cars on the 3rd Street. (像命令)
All carrs should be parked on the 3rd Street. (主詞藏起來委婉、禮貌多了)
