Names and Definitions

     Much nonsense has been written about subordinate and main clauses. Thus, there is the definition of a main clause as "a clause that can stand alone." Teachers who use that definition have not thought very much about what they are doing. "He is a coward" can stand alone, but in "Bill thinks he is a coward," it is a subordinate clause. Some teachers argue that there is an ellipsed "that" in front of the "he" when it is a subordinate clause, but once again they are arguing backwards. They know when to put the ellipsed "that" in only because they know that it is a subordinate clause. Students who cannot distinguish main from subordinate clauses have no way of telling when there is or when there is not an ellipsed "that."
     The idea that a main clause can "stand alone" is probably behind the names "independent" and "dependent" clauses. Although they usually don't realize it, the people who use these terms have the situation backwards. Using our previous example, they would say that "Bill thinks" is the independent clause and that "he is a coward" is dependent. But that doesn't make sense. "He is a coward." is a totally satisfactory, INDEPENDENT statement, whereas "Bill thinks," in our example, DEPENDS on "he is a coward" for its completion. (In their defense, I must note that some of the teachers who use these terms would consider "Bill thinks he is a coward." is the "independent" clause. That makes more sense, but it still does not justify calling "he is a coward" "dependent.")
     Although some textbook writers are too dull be able to follow the logic (See MIMC.), the terms "main" and "subordinate" make much more sense. The prefix "sub-" means "under" (as in "submarine"), less important (as in "subordinate"), or a smaller part of (as in "subassembly"). Subordinate clauses are less important (in the sense that they convey less important information) subassemblies within a main clause. Note that this does not mean that subordinate clauses are so unimportant that they can be eliminated from sentences. In relation to a car, an engine is a subassembly, but there is no functioning car without an engine. As subassemblies, subordinate clauses "always" function as nouns, adjectives, or adverbs within another clause.

     Noun clauses can function in the same way that any regular noun can. For example:

     Object of a Preposition:

They talked {with [whomever they met]}.

Sarah said [she would be late].

Rockefeller gave [whomever he saw] a dime.

[That you should believe me] is not always true.

    Adjective clauses, more properly called "adjectival," function in any way that a regular adjective can. They usually begin with a "who," "whom," "whose" "which," or "that" and immediately follow the noun or pronoun being modified. The "who," "whom," "whose" "which," or "that" function simultaneously as subordinate conjunction and as a subject, object, etc. within the subordinate clause:

We saw the house (DO) [that was destroyed {by the fire}].

She is the one (PN) [who won the race (DO)].

That is the man (PN) [whom (DO of "saw") I saw].

[Note that this use of the objective case ("whom") is apparently dying. Many people would say "... who I saw." Whether or not it is worth fighting to save it is a matter of debate.]

He met the boy (DO) [whose (adj. to "essay") essay won the contest (DO)].

     Adverbial clauses function in the same way that normal adverbs do, but they greatly expand the range of logical relationships that can be communicated. Perhaps the greatest loss from the widely-spread abandoning of traditional grammar in our classrooms is the loss of focus on the functions of adverbial clauses. Instructors from other disciplines have complained to me that, whatever question they ask  their students, too many students respond as if it were a "What?" question. If they ask "How?", if they ask "Why?", if they ask "Under what circumstances?", students respond to the question as if it asked for "What?" Focussing on the functions of adverbial clauses might help to solve this problem.
     Adverbial clauses always begin with a subordinate conjunction, usually a conjunction that expresses some kind of logical relationship between the idea expressed in the adverbial clause and the word being modified.

     Temporal Relationships

[When they were in Paris], they saw the Air Show.
They were in Paris [when they saw the Air Show].

     In his Understanding Grammar, Paul Roberts includes a separate category for "Clauses of Attendant Circumstances," but I cannot see any logical difference between this category and Temporal Relationships. In effect, the subordination of any main clause into an adverbial clause of time creates a clause of attendant circumstances.

Flowers don't grow well [where the soil is rocky].

The boy looks [as if he is pointing at something].
He draws [like a two-year-old draws].

[Prescriptive grammarians claim that the second example contains an error, and that "as" should be used instead. They have lost the battle.]

     Adverbial clauses of degree are crucial to comparison / contrast thinking, which is, as most college Freshman composition teachers will probably note, not a strong point of most students' essays. Perhaps the problem can and should be addressed at the sentence level as well as at that of the essay?

Cats are easier pets to take care of [than dogs are].
William is as slimy [as a snake *is slimy*].

     Relationships of Cause, Effect, and Purpose

     The tendency of traditional grammar to focus on individual constructions usually results in clauses of cause, of effect, and of purpose being treated separately, but they shouldn't be. A "cause" is a "purpose," and every "cause" has at least one "effect." Likewise, "effects" have causes, and may be the result of someone's "purpose." Often a clause that expresses cause:

can be revised to express effect:

Here again, I would suggest that the MIMC principle comes into play. 
     The primary differences between clauses of cause and clauses of purpose are that (1) purpose usually involves an animate will, and (2) in the clause of cause, the effect is known and real, whereas in the clause of purpose, the effect is intended but not necessarily realized. Consider, for example, the following sentence which Roberts (Understanding Grammar, p. 324) offers as an example of "purpose":

Sam put a new lock on the boathouse [so no one would steal his boat].

     Relationships of Condition

     Adverbial clauses of condition establish a simple logical relationship in which the statement in the main clause is asserted to be true only if the statement in the subordinate clause is true.

[Unless it rains], we'll have a picnic.
We'll get a new car, [if we can afford it].

     To "concede" means to "yield" or "admit to be true." An adverbial clause of concession admits the truth of the assertion in the subordinate clause while, contrary to logical expectations, also asserting the truth of the main predication:

[Although Richard Cory was rich], he was unhappy.
[Though it rained], we had the picnic.

     Almost every semester, students ask  me if one can have a subordinate clause within a subordinate clause. The answer is a definite "yes."  See, for example, the example of how to find clauses.

     Parts of subordinate clauses are often ellipsed, thereby reducing the length of the clause. As we will see in Level Five, this reduction may lead to other constructions, but when the subordinate conjunction remains, we have a semi-reduced clause:

[When in Rome], do as the Romans do.

[When *you are* in Rome], do as the Romans do.

    Interjections are one of the seven concepts explored in Level Five. I mention it here simply because at least one of the exercises includes several subordinate clauses used as interjections. Because the concept is not very difficult, you may wish to look at it now. The answer keys to the exercises will include links to an explanation, but if you miss any of these, you should NOT count them as errors. See also "Why Can't I Use 'I'?" and "First-Person Clauses as Interjections"