Some examples of simple, compound, complex and compound-complex sentences using the examples given are the following:
People get so excited about football. I don't understand it. It is not nearly as interesting as baseball. Here we have three simple sentences.
People get so excited about football and I don't understand why; it is not nearly as interesting as baseball. Compound sentence formed by three independent clauses. The firs two are linked by the coordinator "and", and the third one is separated by a semicolon.
I don't understand why people get so excited about football. It is not nearly as interesting as baseball. Complex sentence formed by a dependent clause embedded in an independent clause, introduced by the adverb of reason "why". Afterwards, We have an independent clause.
I don't understand why people get so excited about football, for it is not nearly as interesting as baseball. Compound-complex sentence formed by two clauses, one independent containing a dependent introduced by the adverb of reason "why", and the other one linked by the coordinator "for".