What is a subordinate or dependent clause?
A dependent or subordinate clause is a group of words that has a subject and verb, but cannot stand alone as a sentence. It should be joined to an independent clause to make sense.
What is a complex sentence?
A sentence made up of an independent clause (which is the main clause) plus a subordinate clause is called a complex sentence.
Example
Although Clara Barton is renowned for her work as a Civil War nurse, most people have heard little about her other accomplishments.
Even though Clara Barton is renowned for her work as a Civil War nurse contains a subject (Clara Barton) and a verb (is), but it is not a complete thought. It depends on the rest of the sentence. Therefore, it is a subordinate clause.
Most people have heard little about her other accomplishments are a major clause because it can stand alone as a complete sentence.
The entire sentence is a complex sentence.
What is an adverbial clause?
A subordinate clause that tells us where, when, how, why, or to what extent something happens is an adverbial clause. It can modify a verb, adjective or adverb.
Adverbial clauses start with subordinating conjunctions.
Here are some common coordinating conjunctions:
because since while before after
although where when whenever wherever
if as if until than so that
Some of these words can also serve as adverbs. They are not subordinating conjunctions unless they join an adverbial clause to an independent clause.
Examples
1. She was never heard of since.
Since is an adverb in this sentence.
2. Did I ask you that before?
Before is an adverb.
Is it a preposition or a conjunction?
Some words (such as since, before, after, until, than) that can be used as subordinating conjunctions can serve as prepositions in other sentences. Analyze the function of the word in the sentence before deciding the part of speech. If the word is followed by a subject and verb, it is a subordinating conjunction. If the word is followed by a noun, its modifiers, and no verb, the word is being used as a preposition in that sentence. (Prepositional phrases do not contain verbs.)
Examples
1. Before her career as a nurse, Clara Barton was a teacher.
Before her career as a nurse is a group of words that works together as a unit to modify the verb in the sentence. It does not contain a subject and verb. It is an adverbial prepositional phrase, and before is a preposition.
2. Before she began her career as a nurse, Clara Barton spent twenty years teaching.
Before she began her career as a nurse contains a subject (she) and a verb (began) but cannot stand alone as a sentence. It is an adverbial clause because it tells us when she spent twenty years teaching.
Where should I put the adverbial phrase in the sentence?
An adverbial phrase can come before the main clause or after the main clause in a complex sentence. If the adverbial clause comes before the main clause, separate it form the main clause with a comma. If the adverbial clause comes after the main clause, the comma is not necessary.
Can I start a sentence with "because"?
You may have been told at some time that a sentence cannot start with the word because. If because is the subordinating conjunction that starts an adverbial clause, it is perfectly fine to start the sentence with because. Just be sure that you follow the adverbial clause with a main clause; do not just use the adverbial clause by itself. Whenever you attempt to have a subordinate clause stand alone as an entire sentence, you have written a sentence fragment.