Whats your favourite sport
the doctor said he could help him.（能力）
醫生說他能幫助他。
lily could swim when she was four years old.（能力）
當麗麗四歲的時候她就會游泳。
at that time we thought the story could be true.（可能性）
那時我們以為所說的可能是真的。
could可代替can表示現在時間的動作，但語氣較為婉轉。例如：
could i speak to john，please？
我能和約翰說話嗎？
could you？在口語中表示請求對方做事。例如：
could you wait half an hour？
請你等半個小時好嗎？
could you please ring again at six？
六點鐘請你再打電話好嗎？
(4) can的形式
只有現在式can和過去式could兩種形式。能表示一般現在和一般過去兩種時態，有時也能表示將來。所有其他時態（包括將來時）須用be able to加動詞不定式來表示。例如：
they have not been able to come to beijing.
他們沒有能到北京來。
日常交際用語
1. that's right.
that's right意為"對的"，表示贊同對方的意見、看法或行為，肯定對方的答案或判斷，其相反的表達是that's wrong.有時that's right.=you are right. that's wrong.=you're wrong.
例如：
(1) "i think we must help the old man.我想我們應該幫助這位老人。"
"that's right."或 "you're right.說得對"。
(2) "ten and one is eleven." "that's right."
"10+1=11"."你是對的?quot;
2. that's all right.
that's all right.意為"不用謝"、"沒關系"，用來回答對方的致謝或道歉。
例如：
(1) "many thanks." "that's all right."
(2) "sorry. it's broken." "that's all right."
3. all right.
all right.意為"行了"、"可以"，表示同意對方的建議或要求。有時還可以表示"身體很好"。
例如：
(1) "please tell me about it." "請把此事告訴我。"
"all right.好吧。"
(2)is your mother all right?你媽身體好嗎？