Confusion about the usage of hěn I am learning Chinese with the Memrize App and noticed that often, to say something like "You are x", you will actually say "nǐ hěn x" instead of "nǐ shì x". 

For example as translated by Google: "Nǐ hěn máng" means "You are busy". This confuse me because an example from the app would be "Tā juéde tā hěn piàoliang" meaning "He thinks she is very pretty". Here, "hěn" is obviously used to reinforce "piàoliang".


Why isn't "Ni shi..." used to say "You are..."? Or is it used like that but 
I just haven't come across it yet?
How do you say "You are very busy". Do I have to use another reinforcing word such as "tai"(I know that "tai" doesn't quite fit, but I haven't learn too many words yet ^.^)?
There are two things that need to be aware of here.


The Chinese word 是(shi) is NOT equivalent to English "is/am/are". The Chinese 是(shi) functions as a "=". So both sides of the equal sign should have the same nature. So "ni shi mang" doesn't make sense.
A Chinese adjective is also a verb! That is, "mang" is already "to be busy"; "pretty" is already "to be pretty". Literally "you are busy" is "ni mang". But "hen" should be added to balance the rhythm of the sentence, and it isn't necessary to mean "very". So, the right sentence is "ni hen mang".


Finally, if you want to emphasize "very busy", it can be "ni fei chang mang".