Questions & Answers about El libro no es muy complicado.
Why do we use muy here instead of mucho?
Does muy ever change its ending to match the noun?
Why does complicado end in an -o?
Because it is describing el libro, which is a masculine, singular word. If we were describing a feminine word, like a test (la prueba), it would change to complicada.
Why does no come before es?
In Spanish, the word no almost always goes directly in front of the verb it makes negative. Here, it goes right before es (is) to mean 'is not'.