I have a problem with using "too" in a sentence. In what situation would you use "too" instead of "so" or "very"? Please include an example to illustrate the difference.
It depends on whether you're talking about formal or informal English.
In formal English, "so" or "very" are neutral, and "too" is negative. All three terms mean that the noun has a lot of the quality under discussion, but "too" also means that having so much is a problem.
To make things more confusing, in informal English, "too" can be used in a positive sense, as hyperbole. So ordinarily, if you say something is "too cute," it means you're criticizing its aesthetics. But if you say, "Oh, my God! That's too cute! I love it!" it just means "very cute."
To sum up:
That picture is very sexy. We're going to sell a million copies!
means you approve of it.
That picture is too sexy. We're going to sell a million copies!
means the same thing but is informal and would only be used in some English dialects. On the other hand:
That picture is too sexy. We can't hang it up in this elementary school!
means that you disapprove of the picture's sexiness.
How can you tell which one the speaker means? Only through context. When in doubt, though, avoid using "too" in this sense unless you disapprove.