In some sentences, you can use think of or think about with no difference in meaning, but there are some differences in usage. In general, think of + object is often used to talk about your opinion about things. For example:
What do you think of Karen's new boyfriend?
I'm not sure what I think of the changes in government policy.
Whereas think about without any object can be used in a more general meditative sense, such as
I sat in the sofa thinking about the weekend.
What are you thinking about?
I don't know if American usage is different on this point.
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