A Compound sentence unites two or more independent clauses using a coordenator (for, so, and etc). A Complex sentence has one independent clause and one or more dependent clauses. And a Compound-Complexunites a Compound sentence with a Complex Sentence.
Since marathons are a challenge for me, (<em>dependente clause</em>)
I signed up to run two races (<em>independent clause)</em>
so that I can prove to myself I can do it. (<em>dependent clause</em>)