In each of the following pairs of phrases, what is the difference between the two?
1a. "Had been able" vs.
1b. "Could have"
2a. "Hadn't been able" vs.
2b. "Couldn't have"
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In each pair, the first phrase is about having accomplished (or not) something in the past:
Before arthritis got to his back, he had been able to change his own oil
Before he got his GED, he hadn't been able to get anything better than work as a laborer
The second phrase in each pair is about opportunity, most typically that was missed:
He could have gotten her autograph if he'd had the courage to walk up and say "hi".
He couldn't have eaten the last piece of cake, he was away at school all day.