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He read messages but he couldn’t / wasn’t able to understand them.
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I could / was able to see him through the window.
With all the perfect forms “be able” is usual.
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Since his accident he hasn’t been able to walk by himself.
Could + perfect infinitive is used for past ability when the action wasn’t performed.
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I could have lent you the money. Why didn’t you ask?
Could + perfect infinitive can be used for past abilities when we don’t know whether the action was performed or
not.
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The money has disappeared. Who could have taken it?
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Tom could have taken it. Only he knew where it was.
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He was able to send it. (He sent it).
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He could have sent it (he didn’t send it or we don’t know whether sent it or not).
Could + perfect infinitive can also express irritation.
