Could not vs was not able to?

When it comes to using the correct phrasing of “could not” vs. “was not able to,” there is often confusion. While both phrases technically mean the same thing, there is a subtle difference in usage. In general, “could not” is used when referring to a lack of ability, while “was not able to” is used to describe a failure to do something. Here are some examples: I could not find my keys. (I was not able to find my keys.) This means that you looked for your keys, but you couldn’t find them. I was not able to find my keys. (I could not find my keys.) This means that you wanted to find your keys, but you couldn’t. As you can see, the two phrases can be used interchangeably, but “could not” is more often used when referring to ability, while “was not able to” is used more when referring to failure. There are, of course, exceptions to this rule. For example, if you’re referring to a specific instance when you were not able to do something, you would use “was not able to.” I was not able to find my keys this morning. In this case, “could not” would not be appropriate because you are referring to a specific instance when you were not able to find your keys. Generally speaking, “could not” is the correct phrasing to use when referring to ability, while “was not able to” is more often used when referring to failure. However, there are always exceptions to the rule, so be sure to choose the phrasing that makes the most sense in the context of your sentence.