"He does not want to say because it is very complicated."
Translation:Anh ấy không muốn nói vì nó 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. :)