could是助動詞 ( helping verb ), 後面動詞原型: could V.
否定式的用法有could not / couldn’t
could用法主要有三種：
當could 是can的過去式時, 表示過去有的能力,可以作的事,但現在沒有了。
1. 比較can/could 的差異：
◎ Ethan can sing well. Ethan現在可以唱得好。
◎ Ethan could sing well. Ethan以前可以唱得好。 (但現在一唱, 鬼哭神嚎=_=lll)
2. 也可以比較現在和過去的差異:
◎ When I was young, I could swim, but now I can’t.
◎ When I was a child, I couldn’t ride a bicycle, but now I can.
could可用在不確定, 但有可能發生的事情。比如說:
A：My phone is ringing.
B：It could be Tom. *可能是Tom打來的,但是不確定。
A：Betty didn’t come to class today. Do you know what happened to her?
B：I’m not sure. She could be sick. *Betty可能生病了,但是不確定。
A：It’s raining outside.
B：I know. It could get worse. *天氣可能會更遭，但是不確定。
could 當開頭的問句是禮貌的請求對方做事：
A：Could you close the windows? It’s so cold in the room.
B：Sure. I can turn on the heater if you want.
(注意喔﹗can在這邊其實有提供建議的意思，不是有能力而已)
(in the washroom)
A：Excuse me. Could you bring me some drinks?
B：No problem. What would you like, coffee or tea?
