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My grandfather could speak five languages 

My grandfather was able to speak five languages.

What is the exact difference, as I know can/ could are used for abilities.

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For general ability in the past, you use could. If the occassion happened once, you use was/were able.

It all depends on context if the action is considered general ability. For punctual past abilities, which happened once, it's often used was/were able to.

I called him but I wasn't able to speak. (... I couldn't speak.)
(It was one time.)

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    This is a thought-provoking answer! I'm not 100% sure I agree: consider "I climbed Mount Everest once, but I wasn't able to get to the top" versus "...but I couldn't get to the top." Both of them sound acceptable to me. And yet you're right that there is a bit of a difference in how we use them. "My grandfather could lift a pony over his head" sounds a little different from "My grandfather was able to lift a pony over his head", like you say. – stangdon Jan 30 '16 at 13:51
  • @stangdon I think the same. The difference is not always clear-cut. I think it probably focuses on how is considered the ability to be general or not. – Alejandro Jan 30 '16 at 13:57
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    @stangdon, I think that the rules may be different for negatives (no surprises there) but for an affirmative version of your sentence, Ustanak's Law works: "I climbed Mount Everest once, and I was able to get to the top" is fine because it's once, but I climbed Mount Everest once, and I could get to the top" is wrong. – JavaLatte Feb 25 '17 at 9:31

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