Question

There was another long pause.The boy's mouth opened (joint into simple sentence)

Answer 1

I think the sentence would be:

The boy's mouth opened on another long pause.

Answer 2

The boy’s mouth opened on another long pause.

EXPLANATION:

The simple sentence has independent clauses. The given two sentences are independent clauses and they give a complete meaning if combined. For example, Shoba is a tall girl. This sentence gives a complete meaning to the subject.

Compound sentences are two independent clauses combined together with but, etc., For example I am tall but my sister is short. In the above sentence even if there is no ‘but’ there are two independent sentences. In complex sentences, there will be one dependant and one independent clause. Example: When I come home, I will eat my lunch.