Final answer:
A 'simple sentence' contains one main idea, whereas a 'compound sentence' combines two or more related ideas using a coordinating conjunction.
Step-by-step explanation:
"Here is a sunken boat" is an example of a simple sentence because it contains one main clause with a complete thought and can stand alone as a sentence. Meanwhile, "The settlers in the westward movement started in the east, and then they moved in wagon trains across trails like the Oregon Trail to the west" is an example of a compound sentence. This is because it contains two independent clauses that are joined by a coordinating conjunction, which in this case is 'and'.
Simple sentences are important in writing as they can help to clearly convey a single idea, while compound sentences are used to connect related ideas, making the text richer and the relationships between those ideas clearer.