The document discusses the three main building blocks of sentences: independent clauses, dependent clauses, and phrases. It provides examples of each and explains that independent clauses can stand alone as simple sentences, while dependent clauses cannot. It also introduces the four types of sentences that can be formed using these building blocks: simple sentences with one independent clause, compound sentences with two or more independent clauses joined with coordinating conjunctions, complex sentences with one independent clause and one or more dependent clauses, and sentences that include phrases.