As you know,we use could for general ability. But if we want to say that somebody did something in a specific situation,we have to use was/were able to or managed to **(not **could ). I know the below sentence is correct grammatically. But why? According the above explanation, we should use managed to or was/were able to.

Problematic sentence:the test was difficult.I could only answer half of the questions.

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    I've never heard the rule you are quoting (but as a native English speaker, I've never been taught it). But I think the answer is that it doesn't apply when you have negative polarity terms (eg "I couldn't do it"). "Only" here is such an item, I think. – Colin Fine May 4 '16 at 15:43
  • Likewise, I've never heard this rule and would not find it to be borne out in common British-English usage. I would quite happily say that I could only manage half the questions, or even, that I could only manage to answer half of them. – Spagirl May 4 '16 at 15:47

"Could" is an auxiliary verb with multiple uses. "Could you get me a glass of water?" (asking someone to do something) "He could grow up to be President" (expressing possibility) "He could not climb the tree." (expressing ability or inability) There may be other uses, as well. Considering the third example, though, it shows that there is nothing wrong with saying ""I could only answer half the questions" to mean "I was only able to answer half of the questions." The only difference is that (in the absence of the first sentence, which sets the time in the past) the former version is subject to the possible interpretation "It is possible that I will decide to only answer half of the questions."

Note that I have said "half of the questions." English usage requires the definite article in this case.

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