• Meaning: An essential or restrictive clause answers the question "Which __(noun)__?" If the noun is singular, it points to one particular person or thing when there is more than one possibility. If the noun is plural or generic, it identifies a particular sub-group of the larger group (SOME). A non-essential clause or phrase adds additional, probably important, information, about an already specified person or thing, or about an entire class of people or things (ALL).
• Punctuation: A non-essential or non-restrictive clause is separated from the noun it modifies and from the rest of the sentence by commas, dashes, or parentheses.
• That: only for RESTRICTIVE clauses
• Omitting the relative pronoun: the relative pronoun can be omitted in restrictive clauses only:
  That's the man whom/that I love. = That's the man I love.
  That's my father, whom I love. (the whom cannot be omitted)
• Reducing clauses to phrases: both restrictive and non-restrictive clauses can be reduced to phrases:
  
  The chair that is made out of wood is new. = The chair made out of wood is new.
  (There's at least one other chair, which is not made out of wood. It's probably not new.)
  My chair, which is made out of wood, is new. = My chair, made out of wood, is new.
  (I only have one chair. It's made out of wood, and new.)
• position: a non-restrictive phrase (reduced from a clause) can be moved in front of the subject of a sentence, if it modifies the subject. If the subject is a personal pronoun, this is the only possible position for an adjective phrase. (The result may look like and mean about the same thing as a reduced adverb clause.)
• Which: can be used in a NON-restrictive clause to modify an entire proposition
  

1. Clauses with NO relative pronoun!

In RESTRICTIVE clauses, these relative pronouns can be omitted IF THEY ARE NOT THE SUBJECT OF THE CLAUSE. (The clause must still have a subject and a verb):

that ........... all (that) he and Marianne could say
who(m) ...... a man (whom) I can really love
which ........ complaints (which) politeness had hitherto restrained

Clauses (restrictive or nonrestrictive) in which who, which, or that is the SUBJECT can be reduced to phrases by omitting the relative pronoun and the part of the verb that agrees with the subject:

a. if the verb phrase begins with a form of the verb be, omit it along with the relative pronoun
b. if the verb phrase does not begin with be , change the verb to the present participle (-ing) and omit the relative pronoun
* in either case, you will be left with a phrase beginning with something that can follow be:
a noun (non-restrictive phrases only) -- often called an "appositive"

Bill Clinton, (who is) the President of the US, is from Arkansas.
an adjective (less common; usually only phrases, not single adjectives)

Clothes (which are) wet from the rain can be hung here.
a preposition

People (who are) from Iowa are especially nice.
a present participle (-ing)

The money (which/that was) lying on the table ... or
The money which/that lay on the table ...
= The money lying on the table was mine.
a past participle ( -ed, -en, etc.)

The money (that had been) placed on the table was mine.
c. if the verb is the main verb have, replace it with the preposition with (instead of changing the verb to having):
A man who has a lot of money isn't necessarily happy.
= A man with a lot of money isn't necessarily happy.

JUST BECAUSE YOU CAN reduce a clause to a phrase DOESN'T MEAN YOU SHOULD!: Notice that when you reduce a clause to a phrase, you LOSE the verb or the part of the verb that indicates the TIME. (Present participles are NOT "present"; past parciples are not "past.") If the time referred to will not be obvious from the rest of the sentence, it would not be a good idea to reduce the clause to a phrase!

Think about how strange this sentences would be if you reduced the adjective clause to an adjective phrase!:

The man who built the bridge is dead.

That is used only in RESTRICTIVE adjective clauses (never in NONRESTRICTIVE clauses)

Of course, that is also used in OTHER ways in English, not only in adjective clauses!:

-- as a demonstrative adjective or pronoun (That is not my book. That book is yours.)
-- to introduce a noun clause or indirect statement: (The idea that men could fly was new.)
-- in adverb clauses: (It was such a hot day that we closed the office.)

NON-RESTRICTIVE clauses only:

Q = "quantifier" (... some children, 3 of whom were crying, came ...)
N = "noun" (... my house, a picture of which I have here, is located ... )

In the first sentence above, 3 is the subject of were crying. The nonrestrictive adjective clause can be reduced to an adjective phrase by either:
... changing the relative pronoun to a personal pronoun (whom to them) and omitting the form of be

some children, 3 of them crying, came ...

... or omitting the of along with the relative pronoun and the form of be) :

some children, 3 crying, came ...

In the second sentence, picture is NOT the subject of the adjective clause. This one cannot be reduced to a phrase.

"WH-words" are also used:

-- in questions (Where are they?)
-- to introduce noun clauses or indirect questions (I don't know where they are.)

Ann Salzmann
Intensive English Institute
University of Illinois