ADJECTIVE CLAUSE

See The Sentence for definitions of sentence, clause, and dependent clause.
A sentence which contains just one clause is called a simple sentence.
A sentence which contains one independent clause and one or more dependent clauses is called a complex sentence. (Dependent clauses are also called subordinate clauses.)
There are three basic types of dependent clauses: adjective clauses, adverb clauses, and noun clauses. (Adjective clauses are also called relative clauses.)
This page contains information about adjective clauses. Also see Adverb Clauses and Noun Clauses
A. Adjective clauses perform the same function in sentences that adjectives do: they modify nouns.
The teacher has a car. (Car is a noun.)
It’s a new car. (New is an adjective which modifies car.)
The car that she is driving is not hers.
(That she is driving is an adjective clause which modifies car. It’s a clause because it has a subject (she) and a predicate (is driving); it’s an adjective clause because it modifies a noun.)
Note that adjectives usually precede the nouns they modify; adjective clauses always follow the nouns they modify.
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B. A sentence which contains one adjective clause and one independent clause is the result of combining two clauses which contain a repeated noun. You can combine two independent clauses to make one sentence containing an adjective clause by following these steps:
1. You must have two clauses which contain a repeated noun (or pronoun, or noun and pronoun which refer to the same thing). Here are two examples:
The book is on the table. + I like the book.
The man is here. + The man wants the book.
2. Delete the repeated noun and replace it with a relative pronoun in the clause you want to make dependent. See C. below for information on relative pronouns.
The book is on the table. + I like which
The man is here. + who wants the book
3. Move the relative pronoun to the beginning of its clause (if it is not already there). The clause is now an adjective clause.
The book is on the table. + which I like
The man is here. + who wants the book
4. Put the adjective clause immediately after the noun phrase it modifies (the repeated noun):
The book which I like is on the table.
The man who wants the book is here.
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C. The subordinators in adjective clauses are called relative pronouns.
1. These are the most important relative pronouns: who, whom, that, which.
These relative pronouns can be omitted when they are objects of verbs. When they are objects of prepositions, they can be omitted when they do not follow the preposition.
WHO replaces nouns and pronouns that refer to people. It cannot replace nouns and pronouns that refer to animals or things. It can be the subject of a verb. In informal writing (but not in academic writing), it can be used as the object of a verb.
WHOM replaces nouns and pronouns that refer to people. It cannot replace nouns and pronouns that refer to animals or things. It can be the object of a verb or preposition. It cannot be the subject of a verb.
WHICH replaces nouns and pronouns that refer to animals or things. It cannot replace nouns and pronouns that refer to people. It can be the subject of a verb. It can also be the object of a verb or preposition.
THAT replaces nouns and pronouns that refer to people, animals or things. It can be the subject of a verb. It can also be the object of a verb or preposition (but that cannot follow a preposition; whom, which, and whose are the only relative pronouns that can follow a preposition).
2. The following words can also be used as relative pronouns: whose, when, where.
WHOSE replaces possessive forms of nouns and pronouns (see WF11 and pro in Correction Symbols Two). It can refer to people, animals or things. It can be part of a subject or part of an object of a verb or preposition, but it cannot be a complete subject or object. Whose cannot be omitted. Here are examples with whose:
The man is happy. + I found the man’s wallet. =
The man whose wallet I found is happy.
The girl is excited. + Her mother won the lottery. =
The girl whose mother won the lottery is excited.
WHEN replaces a time (in + year, in + month, on + day,...). It cannot be a subject. It can be omitted. Here is an example with when:
I will never forget the day. + I graduated on that day.=
I will never forget the day when I graduated.
The same meaning can be expressed in other ways:
I will never forget the day on which I graduated.
I will never forget the day that I graduated.
I will never forget the day I graduated.
WHERE replaces a place (in + country, in + city, at + school,...). It cannot be a subject. It can be omitted but a preposition (at, in, to) usually must be added. Here is an example with where:
The building is new. + He works in the building. =
The building where he works is new.
The same meaning can be expressed in other ways:
The building in which he works is new.
The building which he works in is new.
The building that he works in is new.
The building he works in is new.
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D. Adjective clauses can be restrictive or nonrestrictive.
1. A restrictive adjective clause contains information that is necessary to identify the noun it modifies. If a restrictive adjective clause is removed from a sentence, the meaning of the main clause changes. A restrictive adjective clause is not separated from the main clause by a comma or commas. Most adjective clauses are restrictive; all of the examples of adjective clauses above are restrictive. Here is another example:
People who can’t swim should not jump into the ocean.
2. A nonrestrictive adjective clause gives additional information about the noun it modifies but is not necessary to identify that noun. If a nonrestrictive adjective clause is removed from a sentence, the meaning of the main clause does not change. A nonrestrictive adjective clause is separated from the main clause by a comma or commas. The relative pronoun that cannot be used in nonrestrictive adjective clauses. The relative pronoun cannot be omitted from a nonrestrictive clause. Here is an example:
Billy, who couldn’t swim, should not have jumped into the ocean.
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E. Adjective clauses can often be reduced to phrases. The relative pronoun (RP) must be the subject of the verb in the adjective clause. Adjective clauses can be reduced to phrases in two different ways depending on the verb in the adjective clause.
1. RP + BE = 0
People who are living in glass houses should not throw stones. (clause)
People living in glass houses should not throw stones. (phrase)
Mary applied for a job that was advertised in the paper. (clause)
Mary applied for a job advertised in the paper. (phrase)
2. RP + OTHER VERB (not BE) = OTHER VERB + ing
People who live in glass houses should not throw stones.(clause)
People living in glass houses should not throw stones. (phrase)
Students who sit in the front row usually participate more. (clause)
Students sitting in the front row usually participate more. (phrase)