"He does not want to say because it is very complicated."
Translation:Anh ấy không muốn nói vì nó rất phức tạp.
Why is it wrong to use "là" in this sentence?
Anh ấy không muốn nói vì nó là rất phức tạp.
Hi there, because Vietnamese language doesn't require the use of "be" when having a subject accompanied with adjective(s). You just say with this structure: Subject + Adjective, it's ok.
For the sentence has "very", you just add "rất" between Subject and Adjective in Vietnamese sentence.
Ex: He is very rich. /Anh ấy rất giàu.
You can read "Tips & Notes" after the lesson Adject.1, it will explain more clearly. :)