i want to know when we have a sentence like ( the sky is blue)
i know that blue is adjective so it cant be an object ( correct me if i am wrong please )
also if we have sentence like ( he is a young man )
young also adjective but can we say that young man is the object
You're right. In that structure blue is called a predicate adjective. It describes the subject. When a noun is involved, it's called a predicate nominative.
- The sky is blue. <predicate adjective>
- He is a young man. <predictae nominative>
Predicate adjectives and predicate nominatives are called "subject complements". They follow linking verbs.