Identify the compound sentence.
a. the rain began to fall, but we did not want to leave.
b. while it rained on the beach, we swam in the ocean.
c. last summer, we decided to drive to the beach.
d. because it had been sunny all week, we did not bring umbrellas.

The compound sentence is letter A: The rain began to fall, but we did not want to leave.  

A sentence basically has a subject, the object of the sentence or doer of the action and predicate, the verb, the adjective, etc. Sentence structures could be simple (one independent clause), compound (two independent clause with coordinating conjunction), complex (a subordinate & independent clause) and compound-complex sentences (subordinate & two independent clause). These include clauses, conjunctions, coherence and balance and even to the number of words you use in your subject and predicate. The benefit of complex or compound sentences is that it could give you more explanation on the subject or topic of the sentence. This gives you a much more understanding on what the sentence is trying to portray or to message to give.