This is a compound sentence, because there are two independent clauses here: 1. Stone bridges are strong + but 2. (strong bridges) are costly to build. The conjunction 'but' connects two independent clauses, and a compound sentence contains at least two independent clauses, whereas a simple sentence has only one.
Elpenor was one of Odysseus' comrades, and happened to be the youngest. This was labelled as part of the road of trials for that fact he had to mourn about this, thinking this was his fault. He had to overcome this.
