Complex sentences are called complex because they are complex, but that doesn’t mean they’re difficult to understand. A great many sentences we say and write in the most common of circumstances every day are grammatically complex. Take, for example, this statement: She’s one of those persons who always knows exactly what to do. What makes this a complex sentence and is it grammatically correct?
English grammar recognizes three basic types of sentences: simple, compound, and complex (some add a fourth, the compound-complex, but for practical purposes, three will get the work of revision done). These types are based on the kind of clause a sentence contains: independent, subordinate, or both. An independent clause combines a subject and verb to make by itself a complete and rounded thought (she’s an entrepreneur); a subordinate clause also combines a subject and verb, but must work in association with another clause to complete its meaning (because she pays me). Looked at logically, every clause we write states one thought, so the better we can identify clauses, the more control we give ourselves over what we (and others) are saying and writing, describing and arguing.
A simple sentence is made up of one and only one independent clause, so whether we call the statement she is a successful entrepreneur an independent clause or a simple sentence depends on whether we are looking at it grammatically or rhetorically, that is, from the point of view of structure or style. A compound sentence merely multiplies independent thoughts, and so comprises two or more independent clauses: she founded the company twenty years ago and it is now very profitable. A complex sentence includes at least one independent clause and at least one subordinate clause: the company is successful because she is a talented leader. In this last example, we can know the clause because she is a talented leader is subordinate in two ways. If we read the clause aloud, it’s obvious that we are in the middle of a larger thought we want to express; and, if we look closely at the word that introduces the clause, we recognize because as a subordinating conjunction, one of a number of words which tell the reader how to logically relate the thought of the subordinate clause to the other thought in the sentence. Here, the subordinating conjunction because introduces the idea of cause, or reason.
We now can understand why the statement She’s one of those persons who always knows exactly what to do is a complex sentence: one independent clause (she is one of those persons) and one subordinate clause (who always knows exactly what to do). Our empirical test of simply listening to a clause will confirm that the second one is subordinate, but we should recognize, too, that it begins with the relative pronoun who. Such clauses are called relative, and relative clauses are always subordinate. And herein lies an important grammatical caution. A relative pronoun, like all pronouns, refers to a noun in some other clause in the sentence, usually (but not always) in the preceding clause. That noun is called the antecedent of the pronoun, and the antecedent governs the number of the verb in the relative clause. So in our example, the antecedent of the relative pronoun who is persons. The noun persons is plural, so the verb of the relative clause, knows, should in fact be the plural form know. The sentence means to say that there are persons who always know exactly what to do and she is one of them.
Our ear might object, but grammar answers to structure, not rhetorical effect. To recognize that is not to denigrate sound, but to appreciate the independent but complementary roles of, so to speak, the grammatical engineering and stylistic design of a sentence. A complex affair at times? Of course, but so are often the thoughts we want to express. And our goal is to match form and content as closely as possible.