What is a predicative clause?

The predicative of a sentence is acted by a sentence, so the sentence acting as predicative is called predicative clause.

The fact is that he was cheated.

This is a sentence with an architecture, the fact is the subject, the predicate is the architecture, and the predicate is that he was cooked. This sentence led by that is a predicative clause.

The difference between object complement and double object depends on the following two sentences.

They elected him president.

I sent him a telegram.

He in the first sentence is the chosen object, but the president is not the chosen object. It is a supplementary explanation of the object he.

He is still the object of sending in the second sentence, and the telegram is also the object of sending.