If a clause can stand alone as a sentence, it is an independent clause, as in the following example:
Some clauses, however, cannot stand alone as sentences: in this case, they are dependent clauses or subordinate clauses. Consider the same clause with the subordinating conjunction “because” added to the beginning:
In this case, the clause could not be a sentence by itself, since the conjunction “because” suggests that the clause is providing an explanation for something else. Since this dependent clause answers the question “when,” just like an adverb, it is called a dependent adverb clause (or simply an adverb clause, since adverb clauses are always dependent clauses). Note how the clause can replace the adverb “tomorrow” in the following examples:
The committee will meet tomorrow.
The committee will meet when the Prime Minister is in Ottawa.
Dependent clauses can stand not only for adverbs, but also for nouns and for adjectives.
A noun clause is an entire clause which takes the place of a noun in another clause or phrase. Like a noun, a noun clause acts as the subject or object of a verb or the object of a preposition, answering the questions “who(m)?” or “what?”. Consider the following examples:
I know Latin.
I know that Latin is no longer spoken as a native language.
In the first example, the noun “Latin” acts as the direct object of the verb “know.” In the second example, the entire clause “that Latin …” is the direct object.
In fact, many noun clauses are indirect questions:
Their destination is unknown.
Where they are going is unknown.
The question “Where are they going?,” with a slight change in word order, becomes a noun clause when used as part of a larger unit — like the noun “destination,” the clause is the subject of the verb “is.”
Here are some more examples of noun clauses:
about what you bought at the mall
This noun clause is the object of the preposition “about,” and answers the question “about what?”
Whoever broke the vase will have to pay for it.
This noun clause is the subject of the verb “will have to pay,” and answers the question “who will have to pay?”
The Toronto fans hope that the Blue Jays will win again.
This noun clause is the object of the verb “hope,” and answers the question “what do the fans hope?”
An adjective clause is a dependent clause which takes the place of an adjective in another clause or phrase. Like an adjective, an adjective clause modifies a noun or pronoun, answering questions like “which?” or “what kind of?” Consider the following examples:
the red coat
the coat which I bought yesterday
Like the word “red” in the first example, the dependent clause “which I bought yesterday” in the second example modifies the noun “coat.” Note that an adjective clause usually comes after what it modifies, while an adjective usually comes before.
In formal writing, an adjective clause begins with the relative pronouns “who(m),” “that,” or “which.” In informal writing or speech, you may leave out the relative pronoun when it is not the subject of the adjective clause, but you should usually include the relative pronoun in formal, academic writing:
The books people read were mainly religious.
The books that people read were mainly religious.
Some firefighters never meet the people they save.
Some firefighters never meet the people whom they save.
Here are some more examples of adjective clauses:
the meat which they ate was tainted
This clause modifies the noun “meat” and answers the question “which meat?”.
about the movie which made him cry
This clause modifies the noun “movie” and answers the question “which movie?”.
they are searching for the one who borrowed the book
The clause modifies the pronoun “one” and answers the question “which one?”.
Did I tell you about the author whom I met?
The clause modifies the noun “author” and answers the question “which author?”.
An adverb clause is a dependent clause which takes the place of an adverb in another clause or phrase. An adverb clause answers questions such as “when?”, “where?”, “why?”, “with what goal/result?”, and “under what conditions?”.
Note how an adverb clause can replace an adverb in the following example:
The premier gave a speech here.
The premier gave a speech where the workers were striking.
Usually, a subordinating conjunction like “because,” “when(ever),” “where(ever),” “since,” “after,” and “so that,” will introduce an adverb clause. Note that a dependent adverb clause can never stand alone as a complete sentence:
they left the locker room
dependent adverb clause
after they left the locker room
The first example can easily stand alone as a sentence, but the second cannot — the reader will ask what happened “after they left the locker room”. Here are some more examples of adverb clauses expressing the relationships of cause, effect, space, time, and condition:
Hamlet wanted to kill his uncle because the uncle had murdered Hamlet’s father.
The adverb clause answers the question “why?”.
Hamlet wanted to kill his uncle so that his father’s murder would be avenged.
The adverb clause answers the question “with what goal/result?”.
After Hamlet’s uncle Claudius married Hamlet’s mother, Hamlet wanted to kill him.
The adverb clause answers the question “when?”. Note the change in word order — an adverb clause can often appear either before or after the main part of the sentence.
Where the whole Danish court was assembled, Hamlet ordered a play in an attempt to prove his uncle’s guilt.
The adverb clause answers the question “where?”.
If the British co-operate, the Europeans may achieve monetary union.
The adverb clause answers the question “under what conditions?”
Written by David Megginson : http://www.writingcentre.uottawa.ca/hypergrammar/claustyp.html