A conjunction brings things together in a sentence. Those things can be single words (nouns, verbs, modifiers, prepositions) or many words (phrases, clauses). Conjunctions live to link: Joining things is what they do. Without them, whenever our sentences contained lists of two or more, those things would pile up and collide. There are three kinds of conjunctions: coordinating, subordinating, and correlative. Why?
Well, it’s really a matter of sentence engineering. If we think of a sentence as a train, with an engine pulling a number of cars, then the conjunctions are the couplers, and different sorts of couplers are required for different sorts of cars.
Subordinating conjunctions connect cars of different shape. They introduce dependent (subordinate) clauses, and they connect them to the rest of the sentence. Most subordinate conjunctions introduce adverbial clauses—dependent clauses the function of which is to describe a verb, adjective, or adverb in a nearby clause. Like all adverbs, these clauses are concerned with when, why, how, and where, and we use different subordinating conjunctions for each of these functions.
The WHY subordinating conjunctions and conjunctive phrases are because, since, as, in order that, so that…
Yoshi has begun to eat his main meal at the office because he’s so tired of Myrtle’s lasagna.
The WHEN: after, before, once, till, until, when, since, whenever, while, as long as, now that…
Whenever she serves him a three-inch cube of lasagna, his eyes glaze over.
The WHERE: where, wherever, whence…
For her part, Myrtle dreams of owning a restaurant where she can serve one hundred distinct pasta dishes.
The HOW (including contrast and hypotheticals): however, although, though, if, as if, even if, lest, supposing, unless, as much as, as though, no matter, provided that, rather than…
At the end of their last blowout, Yoshi vowed to move into the basement if Myrtle refused even to try preparing shrimp tempura.
Not all subordinating conjunctions are adverbial in nature. One subordinating conjunction introduces a noun clause: that.
When that is used as a subordinating conjunction (rather than as a pronoun, as in That was unacceptable!), it is often omitted and understood. The sentences
Myrtle’s greatest flaw is that she always has to be perfect.
Myrtle’s greatest flaw is, she always has to be perfect.
--mean the same thing. Whether that is visible in the sentence, it is still there, acting to join the noun clause to the rest of the sentence.
WARNING: Transitional adverbs are often confused with conjunctive adverbs—and no wonder! The two terms sound interchangeable—but they are not. These words include however,* moreover, nevertheless, consequently, as a result…
The basic function of these words is to transition from sentence to sentence, from independent clause to independent clause:
Nowadays, Myrtle has become an authority on the cuisine of the Sandwich Islands. Moreover, she has written a bestseller on the subject; as a result, she is raking in serious money. However, and rather ironically, she does not know how to make an actual sandwich.
When we mistake one of these common expressions for a subordinating conjunction, we commit comma splices, such as Yoshi secretly loves clam cakes, nevertheless he forces himself to eat steamers. Only conjunctions can join clauses. Transitional adverbs cannot.
So: In order for our complex sentences (sentences including at least one independent clause and one dependent clause) to work, we need subordinating conjunctions. Also, we must not confuse them with transitional adverbs. While transitional words do strongly imply connections between ideas, they do not literally join them together. To join, you need a conjunction.
 The fact that however can, in fact, be used as a subordinating conjunction may just confuse us further. But if we compare sentences using however as a subordinating conjunction with sentences using it as an adverbial transitional conjunction, the confusion should dissipate:
Myrtle vowed to please Yoshi’s palate however she could. [however joins the dependent clause “she could” to the rest of the sentence]
Yoshi was grateful for her willingness to meet him halfway; however, he still sometimes snuck off to the sushi bar on Sunday afternoons. [however transitions between one independent clause to another]
Written by Jincy Kornhauser at Pearson Tutor Services