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I encountered a question at the end of a unit about how to use “think about” and “think of”. It’s stated that we use “think about” when we consider an idea or concentrate on it, and we use “think of” when an idea comes into our mind. The question is a fill-in question:

I don't really want to go out with Tom tonight. I'll have to … an excuse.”

I think we have to put “think about”, because we have to consider or concentrate on an excuse. But in the answer section, “think of” is stated as the answer. Which one is grammatically correct? Isn’t “think of” wrong? Why?

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The answer is given in the question itself:

We use “think of” when an idea comes into our mind.

If you don't want to go out with Tom, then you need to come up with an excuse to not do so. To come up with an excuse, you need to wait for one to "come into your mind."

If you are following the explanation from the unit, that's the sense of what's going on:

“I don't really want to go out with Tom tonight. I'll have to think of an excuse.”


On the other hand, if you already have an excuse, and are pondering if you should use it or just tell Tom you don't want to go out, then think about would be more relevant. In that context, you are debating if you want to employ the excuse you've already come up with.

But in that scenario, the example sentence would be constructed differently.

One possible way of rephrasing it to use this sense follows:

“I don't really want to go out with Tom tonight, but I want to be honest with him too. I'll have to think about the excuse I had prepared.”

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The better fit for that use is think of, and that sense is defined here:
Cambridge Dictionary "think of"

to produce a new idea or plan: We'll have to think of a pretty good excuse for being late.

There are several other examples of that use at the dictionary link.

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Here think of should be the answer as you're not supposed to think about the excuse but you are thinking of an excuse.

Thinking about the excuse is like thinking whether an excuse is suitable or not. but thinking of an excuse is like you are just making up an excuse.

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