Today's tip: 表示过去的能力

Welcome to Daily Tips on Learning English. Today’s tip is on expressing ability in the past.
There are two ways of expressing ability in English. One, “can” or “could”. Two, “be able to”. In negative sentences, there’s no difference in meaning. So, “I couldn’t lift the piano” and “I wasn’t able to lift the piano” mean the same thing. However, in affirmative sentences about past ability, “could” usually means “used be able to”. The use of “could” usually indicates that the ability existed in the past, but does not exist now.

For example, “When I was young, I could run fast” means that I can not run fast now. On the other hand, if the speaker is talking about an ability to do something at one particular time in the past, “was/were able to” can be used in affirmative sentences, but “could” cannot.
For example, your car broke down. It stopped working. So you took it to get fixed two days ago. When I saw you today, you told me that the mechanic fixed your car. If you said, “The mechanic could fix my car yesterday”, that sentence is not correct. You must say, “The mechanic was able to fix my car yesterday” or “The mechanic managed to fix my car yesterday.”

Let’s look at another example. I have been looking for a CD I like for a long time. I could not find it. I was not able to find it for a long time. But yesterday I was able to find it. Yesterday I managed to find it. But you cannot say, “Yesterday I could find it.”

Remember “I could not” and “I was not able to” are the same, but “I could” and “I was able to” are not the same. “I could” means I used to be able to, but now I’m not able to. “I was able to” means I have the ability at one particular time in the past.
This has been today’s daily tip on learning English. Tune in tomorrow for another tip.