When to use "Very" or "Too"?

As a non-native English speaker, this is a very common question. Both words are written before adjectives or adverbs and both words use is to remark and intensify a meaning. The key is the contexts. While the word "very" doesn't have context, the word "too" is used in negative contexts.

• I think I failed the exam. It was too difficult
• I think I failed the exam. It was very difficult

In this example, failing this exam is a bad thing. Therefore, we shall use too instead of very.

• You are very smart
• You are too smart

Tricky one, I know. It really depends on your contexts. If you are compliment a friend about something like he was the only who passed an exam, then you shall use "very" as you compliment him. But if you are talking to a person who takes advantage of others using his intelligence, and you are just signaling the fact with disagreement, then you should use "too".

As a rue of dumb, use "very" for positive or neutral things and use "too" for negative.

Good luck!