There are three basic kinds of dependent clauses:
A subordinating conjunction is used to introduce and connect a subordinate clause to a verb, verbal, or modifier in another clause. The only part of speech that can modify all three of these is an adverb, so subordinate clauses are, by and large, adverbial. (See below for more info.)
Here's a useful list of subordinating conjunction. Take note, however, that some of these can be used as other parts of speech as well:
A subordinate clause beginning with a subordinate conjunction answers a question such as “Why?” or “When?” or “How?” which makes it adverbial.
In the example above, the subordinate clause joins a dependent idea (the play being publicized well in advance) in contrast to the main clause. Because the subordinate clause explains the reason opening night was poorly attended, it therefore modifies the entire main clause in answer to the question "Why?"
A subordinating conjunction should not be misidentified as a Relative Pronoun, a special pronoun that begins an adjective clause. Both, subordinating conjunctions and relative pronouns, create dependent clauses, but in radically different ways.
For the grammar novice, the fact that many of the subordinating conjunctions listed above also have counterparts as prepositions may come as a “Yikes!” moment. For example, the word “after” is a subordinate conjunction, as in "after we go to the movie”; however, it's also a preposition, as in “after the movie”. (It's also a conjunctive adverb, as in "After, they all went to the movies together," but I'll try not to incite panic quite yet.) Teachers might argue, the difference between a phrase and a clause is clear: a clause always has a subject and predicate.
True enough, but what about prepositional phrases containing adjective clauses connected to their noun objects, or adverb clauses connected to their verbal objects such as infinitive phrases or gerund phrases? At some point, it can all become just a mishmash of grammar. Try sorting out the following sentence:
It’s not a particularly unusual sentence, yet a litany of verbal phrases, more prepositional phrases, and even a relative clause almost hide the fact that there’s a subordinate clause in this sentence, which begins with the subordinate conjunction, “although.” Check out the diagram:
Beginners may not be up to the task of diagramming such a sentence, but they can still learn to identify the hallmarks of a subordinate clause when they read it or write it:
In addition to prepositions, students should be on the lookout for certain conjunctive adverbs misused as subordinating conjunctions, typically causing comma splices. A conjunctive adverb is a special adverb that starts a sentence in the manner of a transitional word. You’re cautioned to flag the following list of conjunctive adverbs for yourself and pay extra special attention to them in your writing:
To diagram a subordinating conjunction, a long, dashed diagonal line connects the two clauses and demonstrates where and how they connect.
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