couldn't vs. couldn't have

Question

I've been recalling some grammar recently and become slightly puzzled. Namely, I can't quite spot a difference between couldn't + verb and couldn't have + verb when referring to the past. For instance:
(1)She couldn't do that.
(2)She couldn't have done that.
What is the difference in meaning between the two statements?
Thanks in advance for help!

Philip · Accepted Answer

1 She wasn't able (perhaps by choice). She couldn't go with us to the movie last night.
2 It was impossible. She couldn't have shot the guy because she was 50 miles away at the time.

CalifJim · Answer

Anonymous(1)She couldn't do that.

1. She was not able to do that. She failed to do that.
2. She would not be able to do that. She would fail if she tried.
3. She was not permitted to do that.

1. We couldn't get in because Jack forgot to bring the key.
2. Look at that gymnast! He's standing on his head. I'll bet you couldn't do that.
3. The journalists wanted to cross the police lines to see the crime scene up close, but the police said they couldn't do that.

Anonymous(2)She couldn't have done that.

She would not have been able to do that. / She would not have been capable of doing that.

4. Jack couldn't have got in because he didn't have the key.
5. She said that Lucy insulted her, but I don't believe it. Lucy could never have done that.
___________________

I suppose the difference between 1 and 4 is the most confusing. 1 just says what happened at the time of the event. 4 makes a comment on the logic of the situation, perhaps to deny someone's claim that Jack got in.

CJ

couldn't vs. couldn't have

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