Question

Add a subordinate clause to expand the sentence meaningfully. He completely forgot.

Answer 1

He completely forgot about the homework given in school

Answer 2

To do so at first you will need to understand that there is a difference between the sentence and a clause where a sentence comprises of a subject and a predicate.

Where clause is that particular part of the sentence that can exist independently. It is used to complement the sentence.

He completely forgot about the project to be submitted in the college.