Where coordination lines up independent main clauses, subordination takes clauses and makes them "subordinate" to the main clause of a sentence.
I have a cat. (independent)
The cat is black. (independent)
I have a cat, and the cat is black. (coordination)
I have a cat who is black. (subordination)
Subordinate clauses often don't seem like they could stand on their own as independent sentences (hence they're often called "dependent" clauses):
*Who is black. *That we love. *Who eats kibble.
Like all clauses, subordinate clauses have (usually) a verb of some form, and frequently subjects and objects, complements, adjuncts and modifiers – often English subordinate clauses have subordinate clauses of their own.
In keeping with the fractal nature of English grammar, subordinate clauses can fill many roles in the sentence that are filled by simpler phrases in simple sentences. We call a sentence that has subordinate clauses in such roles "complex," as opposed to "simple," sentences.
NOMINAL CLAUSES take the place of noun phrases and play similar roles in sentences:
Subject:What Whisper likes is brushing. What Whisper eats makes Gemma jealous. What Gemma doesn't know won't hurt her.
Direct Object: I know what Whisper did last night. I considered what you told me. I want what you want.
Complement of Adjective: I'm sure that Whisper ate his kibble. I'm afraid he also ate a mouse.
Complement of Preposition: There's no time for what you want to do. The box was sitting about where I thought it would be.
RELATIVE CLAUSES usually postmodify nouns or noun phrases. They're introduced by the relativizer "that," by wh- words, or sometimes by nothing at all (the zero relative), as in the following three cases:
Whisper is a cat that likes people.
We gave Soames to a woman who loves cats.
Soames was a stray kitten we found.
Like many kinds of modifying phrases and clauses, relatives can be "restrictive" or "non-restrictive." Restrictive relative clauses are vital to the information conveyed in the sentence, and in text they are not set off by punctuation. Non-restrictive relative clauses are more supplemental in meaning, and are set off by commas (in most cases) or other punctuation:
RESTRICTIVE RELATIVE CLAUSES:
Give some food to the cat who needs it most.
The cat I like best is Whisper.
A person that hates cats shouldn't own any.
NONRESTRICTIVE RELATIVE CLAUSES:
Soames, who we found on the curb, became our favorite kitten.
Whisper only eats IAMS, which is an expensive brand of catfood.
Whisper (who had won the primary) was voted Best Cat.
ADVERBIAL CLAUSES should be familiar from some of our exercises on adjuncts: they are set off from the core of the sentence and add further meaning or information:
While I was sleeping, Whisper stole away out the back door.
Whisper ran for the fence when he saw the hen.
I let Whisper in through the window, since I couldn't sleep anyway. Gemma, however she managed it, squeezed through the pet door. Because I could not stop for Death, he kindly stopped for me.
A very common kind of subordinate clause, sometimes considered as an adverbial clause, is the CONDITIONAL CLAUSE introduced by "if":
If Whisper is hungry, I will give him some Fancy Feast.
You can have a kitten if I find another one.
COMPARATIVE CLAUSES are introduced by "than" or "as" and, as the name implies, compare the information in the main clause to something else:
Gemma is fatter than Whisper is.
Whisper runs as fast as most possums can run.
All the clauses we've looked at above have been FINITE CLAUSES. In a finite clause, the introductory word (that, a wh- word) takes the place of a subject (or object); or sometimes a subject is present after the relativizer or zero, or after "if," "than," "as," or various subordinators like "since" or "because.) A verb is present and shows tense (past or present) and can show other verb features like aspect and modality. But in
NON-FINITE CLAUSES, the subject may or may not be present, and the verb takes one of four forms that don't inflect in any way:
Whisper, riding a skateboard, flew past.
Did you see Whisper wearing that sweater?
I don't see many cats eating broccoli.
Eating fresh fish is every cat's right.
He had not worries about killing the mouse. Do you approve of me getting another cat?
All things considered, Whisper is the paragon of cats.
Whisper, soaked to the bone, cried and meowed.
A cat satisfied with the food will not seek a new home.
I want Whisper to catch that brown rat.
To be honest, I prefer cats to dogs.
You must wake up early to outsmart Whisper.
BARE INFINITIVE clauses:
Whisper's antics make me want to smile.
Whisper helps maintain a mousefree house.
Help her pack that suitcase.
Note how the subject of a non-finite clause, when it's a pronoun, is in the objective case, as opposed to the bare case in a finite clause. Contrast:
Did you see us catching that fish?
I took a picture of him jumping over the wall.
It wasn't easy for him to jump that high.
After we caught the fish, we broiled them.
The wall that he jumped over was four feet high.
The heights he jumps frankly scare me.
Note that the -ing and -ed participles, and the "to" infinitive, have other uses that get pretty far from the typical verb functions into the realm of nouns and adjectives. A partial list: