A conjunction joins words and groups of words.

There are two classes of conjunction: co-ordinate or coordinating and subordinate or subordinating.

Co-ordinate conjunctions: and, but, either…or, neither…nor.

Subordinate conjunctions: that, as, after, before, since, when, where, unless, if.

Mother and Father are driving me to New Orleans. (and is a coordinate conjunction joining words of equal significance in the sentence.

I painted the walls but Jack painted the woodwork. (but is a coordinate conjunction joining clauses of equal significance in the sentence. Either clause could stand alone as a sentence.)

Since you can’t get away, we’ll go without you.
(Since is a subordinate conjunction joining a less important thought to a more important thought. The main clause, we’ll go without you, can stand alone as a complete thought. The subordinate clause, Since you can’t get away, is an incomplete thought. It is dependent upon the main clause for meaning.)

NOTE: The relative pronouns who, whom, which, and that are used in the same way that subordinate conjunctions are. The difference is that the relative pronouns serve three purposes at once:

1) they stand for a noun in the main clause
2) they connect the clauses
3) they serve as a subject or object word in the subordinate clause:

He is the man who invented the hula hoop. (who stands for man and is the subject of invented)

Charles is the boy whom the other children tease. (whom stands for boy and is the object of tease)

Give me the piece of string that is waxed. (that stands for string and is the subject of is waxed)

There goes the horse which won the Derby. (which refers to horse and is the subject of won)

The possessive adjective whose can also be used to join clauses:
That’s the bird whose plumage I admire. (whose refers to bird and describes plumage)

How the Three Types of Conjunctions Connect Ideas

This post defines and discusses the three types of conjunctions (words or phrases that serve as a bridge linking two words, phrases, clauses, or sentences): coordinating, correlative, and subordinating conjunctions.

Coordinating Conjunctions

Coordinating conjunctions, also called coordinators, join words, clauses, or sentences of equal importance. The most common coordinating conjunctions, frequently listed in the following order to reflect the use of the mnemonic FANBOYS, include for, and, nor, but, or, yet, and so. Others are neither, only, and “no more,” as well as several British English conjunctive phrases that combine one of the six conjunctions besides nor with that one (such as “and nor”).

Examples of the ten listed conjunctions in use, accompanied by a description of their functions, follow:

I walked, for I was in need of exercise. (rationale)
I walked, and I ran. (addition)
I neither walked nor ran. (alternative without contrast)
I walked, but I ran, too. (contrast or exception)
I (either) walked, or I ran. (alternative with contrast)
I walked, yet I ran, too. (contrast or exception)
I walked slowly, so I ran to catch up. (consequence)
I don’t walk; neither do I run. (addition)
I don’t walk; no more do I run. (addition)
I would walk, only I run. (contrast)

Note that the distinction between nor and or, which are generally accompanied, respectively, by neitherand either (although the latter is parenthesized in the pertinent example because it is optional), is that with neither/nor, the choices do not affect each other, whereas with either/or, one choice cancels the other out. But and yet are virtually interchangeable, while the three addition conjunctions, and, neither, and “no more,” are listed in order of formality, with “no more” generally restricted to ritualistic or poetic usage. Only is used in the sense of “That [one idea] would be true if this [another idea] were not.”

Correlative Conjunctions

Correlative conjunctions include, among others, the following word or phrase pairs, which function to compare two pairs of words or phrases in a sentence that have equal weight; each is followed by an example:

as much/as: Vacations like that can be a pain as much as they are a pleasure.
as/as: This party is as dull as I expected it to be.
both/and: Both the car and the truck are new.
either/or: Either go now, or wait until later.
just as/so: Just as you enjoy going to the theater, I like watching movies.
neither/nor: Neither my father or my mother was born here.
no sooner/than: No sooner had she read the letter than he arrived.
not/but: It is not me but her who is to blame.
not only/but also: I am not only tired but also angry.
rather/than: I would rather play tennis than golf today.
the/the: The more you read about it, the greater a problem it seems to be.
whether/orv We couldn’t tell whether the baby is a boy or a girl.

Some sentences incorporating correlative conjunctions are easier to construct than others. Those involving either/or and neither/nor, and “not only”/“but also,” are often erroneously composed because the first word or phrase is incorrectly situated in the syntax of the sentence.

Note, for example, that in the following sentence, the placement of “either be able to” suggests that “carry on” will have a counterpoint later in the sentence: “Smith should either be able to carry on investing via his equity plan or by using the tax shelter within the new savings account.” However, the counterpoints, separated by or, the second of the two correlative conjunctions, are “his equity plan” and “using the tax shelter,” so either should immediately precede the first of the two choices, just as orimmediately precedes the second choice: “Smith should be able to carry on investing either via his equity plan or by using the tax shelter within the new savings account.”

Likewise, in “People did not only see him as a great athlete but also as a great man,” the suggestion is that people did two things in relation to the subject, including seeing him and something else. However, the intended meaning is that they saw him in two contexts, so “not only,” like “but also,” applies to saw and should therefore follow it, while “not only” immediately precedes “as a great athlete,” just as “but also” immediately precedes “as a great man”: “People saw him not only as a great athlete but also as a great man.”

Subordinating Conjunctions

Subordinating conjunctions, which join independent clauses and dependent clauses, or introduce adverbial clauses, include, among others, the following words and phrases:

after
although
as
“as far as”
“as if”
“as long as”
“as soon as”
“as though”
because
before
“even if”
“even though”
“every time”
if
“in order that”
once
“provided that”
“rather than”
since
so
“so that”
than
though
unless
until
when
whenever
where
whereas
wherever
while
why

An adverbial clause is the beginning of a sentence such as “After searching the desk, I checked the file cabinet.” The same sentence can be inverted so that the subordinating conjunction links the independent clause “I checked the file cabinet” and the dependent clause “searching the desk.”

Filling a similar role are conjunctions technically known as complementizers, such as that and whether, which turn a clause into a sentence’s subject or object. Examples include that in “John said that she was going to be here” (although that as a complementizer is generally optional) and whether in “I don’t know whether I can attend.” (Here, as in many but not all usages, if is interchangeable with whether.)