What's the difference between "Think of", "Think on" and "Think about"?

Question

Answer

Think about es lo regular.
Think of es como más espontáneo, cuando algo se te ocurre, pero también se usa como think about. 
Think on es más como reflexionar sobre algo.

Answer

I was going to say that "think of" and "think about" are different, and sometimes they are, but they are often interchangeable. "Think on" is a slang or regional variation of "Think about". People don't use it where I live.

I will give a couple examples where I would use one but not the other, if that helps.

"To think about (something)" means to ponder, consider or decide something. 
Examples: 
"Let me think about it." [Give me some time before I decide.]
"What do you think about when you have nothing else to do?"

"To think of (something)" often means to bring something to your mind. It implies less deep thinking. 
Examples:
"Think of a number between one and ten."
"Let me think of something." [Give me some time to find an answer, plan, or solution.]

But there are many times when both "of" or "about" are used.
"She's thinking about/of quitting his job."
"What do you think about/of that person/book/etc.?" [what is your opinion?]

In general I would say that if you use them carefully (which not everyone does), "Thinking about something" implies more careful consideration, while "Thinking of something" implies that something entered your consciousness, although you might not have considered it carefully.

Example: 
"After their argument, she thought of leaving her husband." [the idea occurred to her.]
"After their argument, she thought about leaving her husband. [this was an idea she spent some time with and maybe she considered it for a long time.]

Answer

I would say they are all about the same. "Thinking about" and "thinking of" are more common though.  "Thinking on" is not very common.

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