Bàn về could (do) và could have (done)

BÀN VỀ COULD (DO) VÀ COULD HAVE (DONE) 
Chúng ta sử dụng could theo một số cách. Đôi khi nó có thể là quá khứ của can 
(Xem chi tiết TẠI ĐÂY): 
- Listen. I can hear something. (now) 
- I listened. I could hear something. (past) 
Nhưng could không chỉ được sử dụng theo cách này. Chúng ta còn có thể sử dụng 
could để nói về những thành động có thể ở hiện tại hoặc tương lai (đặc biệt là khi 
đưa ra gợi ý). Chẳng hạn: 
- A: What shall we do this evening? 
- B: We could go to the cinema. 
- It's a nice day. We could go for a walk. 
- When you go to New York next month, you could stay with Barbara. 
- A: If you need money, why don't you ask Karen? 
- B: Yes, I suppose I could. 
Can cũng có thể sử dụng được trong những câu trên ('We can go for a walk.' etc.). 
Could mang tính ít chắc chắn hơn can. 
Bạn phải sử dụng could (không dùng 'can') khi bạn không thực sự hàm ý những gì 
bạn nói. Ví dụ: 
- I'm so angry with him. I could kill him! (không nói 'I can kill him') 
Chúng ta cũng sử dụng could để nói rằng một thứ gì đó có thể bây giờ hoặc trong 
tương lai: 
- The phone is ringing. It could be Tim. 
- I don't know when they'll be here. They could arrive at any time. 
Can is không thể sử dụng trong những trường hợp như vậy (không nói 'It can be 
Tim'). Trong những câu như vậy could có nghĩa gần giống với might: 
- The phone is ringing. It might be Tim. 
So sánh could (do) và could have (done): 
- I'm so tired. I could sleep for a week. (hiện tại) 
- I was so tired. I could have slept for a week. (past) 
Rất nhiều khi chúng ta sử dụng could have (done) để nói về những điều có thể 
nhưng không xảy ra: 
- Why did you stay at a hotel when you went to New York? You could have 
stayed with Barbara. 
(= you had the opportunity to stay with her but you didn't) 
- Jack fell off a ladder yesterday but he's all right. He's lucky - he could have hurt 
himself badly. 
(but he didn't hurt himself) 
- The situation was bad but it could have been worse. 
Đôi khi could có nghĩa là 'would be able to': 
- We could go away if we had enough money. 
(= we would be able to go away) 
- I don't know how you work so hard. I couldn't do it. 
Could have (done) = would have been able to (do): 
- Why didn't Liz apply for the job? She could have got it. 
- We could have gone away if we'd had enough money. 
- The trip was cancelled last week. Paul couldn't have gone anyway because he 
was ill. 
(= he wouldn't have been able to go) 
- You did very well to pass the exam. I'm sure I couldn't have passed it. 
(= I wouldn't have been able to pass it if I had taken it)