A colon is also used to add more information and especially to give examples of something in the sentence. A semi-colon is used to connect clauses and is the most important punctuation mark for combining sentences. Remember, it is the semi-colon and not the conjunctive adverb that connects the clauses. Coordination is a way of adding sentences together. One clause is not more important than the other grammatically.
Subordination is a way of combining sentences that makes one sentence more important than the other.
We can reduce it to less than clause. The boy was scared by the movie. He studied for the test. She saw the movie star in the restaurant. There was a story in the newspaper.