Let’s turn our attention to the sentence structure that relative pronouns create. To speak of sentence structure implies the idea of building, constructing, remodeling, and if we think for a moment about those associated ideas, we can see how appropriate they are to the work of writing: to build something, an addition to a house or a sentence in a paragraph, means we construct by design, and if the plans we draw up in a draft don’t work as we expected, we can rebuild it, change its design and function, by moving elements around to better effect. Thus the importance of being able to recognize constructional elements, both material and grammatical.
The relative pronouns are one such constructional element in a sentence, and the major forms are who, which, and that. The term relative is of Latin origin and means to refer back (our relatives are called our relatives because we refer back to them genetically or socially), and like almost all pronouns, relative pronouns refer back to what is called an antecedent, the word for which they are acting as a substitute in another clause. In the sentence The tree that fell down in the storm was 100 years old, the relative pronoun that refers back to its antecedent tree and is standing for that noun as subject in the clause that fell down in the storm. What can make this tricky to see is the fact that English interfolds one clause within another, a common arrangement that can be both grammatically challenging and stylistically interesting.
And this really is why we should care about understanding this ubiquitous part of speech. Understanding the grammar that connects words meaningfully in a sentence gives us a way to change sentences that don’t work. Writing clearly always begins with an awareness of our audience: who is likely to read our work and how much do they likely know already about the subject? These answers will determine our written relationship with our readers, a relationship which includes our word choice and the complexity of our sentence structure. It can be helpful to remember that we can fail to communicate our ideas just as easily by writing too simply as by writing too complexly, and so being aware of sentence structure gives us a way to modulate our statements more easily to fit the circumstances.
Relative pronouns always begin a new clause called, appropriately enough, a relative clause. We know already that there are two kinds of clauses, independent and subordinate; the relative clause is always a subordinate clause. So when we read that the tree that fell down in the storm was 100 years old, we know which tree was 100 years old because of the relative clause that fell down in the storm. Those words specify the 100 year old tree in question, and to specify or identify is the work of an adjective—all of which means that the relative clause that fell down in the storm is an adjective modifying the antecedent tree. The clause in its entirety is an adjective, and has to interrupt the independent clause (the tree was 100 years old) in order not to be too far from the word it modifies. This arrangement of beginning an independent clause (the tree), then inserting another clause (that fell down in the storm) before the first clause has a chance to complete itself (was 100 years old) is very common in English.
Statements that comprise both an independent and a subordinate clause are called complex sentences, and with good reason. This interrupting arrangement of clauses can make things difficult for readers when the topic is involved and unfamiliar, and so it can help to remember that we always have the option to redesign a complex sentence as a compound one. A compound sentence contains two or more independent clauses (no subordinate ones), and so it generally simplifies a statement: That tree was 100 years old and it fell down in the storm. There is certainly a stylistic different between the compound and complex versions, with gains and losses on both sides. What is at issue here, though, iss simply to see that we have a choice, and that it is the understanding of grammatical structure that reveals the existence and execution of that choice to us.