Clauses

In grammar, a clause is the smallest grammar unit that can express a complete proposition. A typical clause consists of a subject and a predicate, where the predicate is typically a verb phrase – a verb together with any objects and other modifiers. However the subject is sometimes not expressed; this is often the case in null-subject languages, if the subject is retrievable from context, but it also occurs in certain cases in other languages such as English (as in imperative sentences and non-finite clauses).
A simple sentence usually consists of a single finite clause with a finite verb that is independent. More complex sentences may contain multiple clauses. Main clauses (= matrix clauses, independent clauses) are those that could stand as a sentence by themselves. Subordinate clauses (= embedded clauses, dependent clauses) are those that would be awkward or nonsensical if used alone.

Independent Clauses

An independent clause (or main clause) is a clause that can stand by itself, also known as a simple sentence. An independent clause contains a subject and a predicate; it makes sense by itself.
Independent clauses can be joined by using a semicolon or a coordinating conjunction (for, and, nor, but, or, yet, so).

Denpendent Clauses

A dependent clause (or a subordinate clause) is a clause that augments an independent clause with additional information, but which cannot stand alone as a sentence. Dependent clauses either modify the independent clause of a sentence or serve as a component of it. Some grammarians use the term subordinate clause as a synonym for dependent clause. Other grammars use subordinate clause to refer only to adverbial dependent clauses. There are also different types of dependent clauses, including noun clauses, relative (adjectival) clauses, and adverbial clauses.

Dependent clauses often begin with a a subordinating conjunction or relative pronoun (see below) that makes the clause unable to stand alone.

Subordinating Conjunctions
after	although	as	because
before	even if	even though	if
in order that	once	provided that	rather than
since	so that	than	that
though	unless	until	when
whenever	where	whereas	wherever
whether	while	why

Relative Pronouns
that	which	whichever
who	whoever	whom
whose	whosever	whomever

For example:
The door opened because the man pushed it.
Dependent clauses can be nominal, adverbial or adjectival.
A nominal clause (noun clause) functions like a noun or noun phrase. It is a group of words containing a subject and a finite verb of its own and contains one of the following: that | if | whether
For example:

• I wondered whether the homework was necessary.

Noun clauses answer questions like "who(m)?" or "what?"


An adverbial clause (adverb clause) is a word or expression in the sentence that functions as an adverb; that is, it tells you something about how the action in the verb was done. An adverbial clause is separated from the other clauses by any of the following subordinating conjunctions: after | although | as | because | before | if | since | that | though | till | unless | until | when | where | while
For example:

• They will visit you before they go to the airport.

Adverbial clauses can also be placed before the main clause without changing the meaning.
For example:

• Before they go to the airport, they will visit you.

!Note - When an adverb clause introduces the sentence (as this one does), it is set off with a comma.
Adverb clauses answer questions like "when?", "where?", "why?"
An adjectival clause (adjective clause or relative clause) does the work of an adjective and describes a noun, it's usually introduced by a relative pronoun: who | whom | whose | that | which
For example:

• I went to the show that was very popular.

This kind of clause is used to provide extra information about the noun it follows. This can be to define something (a defining clause), or provide unnecessary, but interesting, added information (a non-defining clause).
For example:

• The car that is parked in front of the gates will be towed away. (Defining relative clause.)

Information contained in the defining relative clause is absolutely essential in order for us to be able to identify the car in question.

• My dog, who is grey and white, chased the postman. ( Non-defining relative clause)

A non-defining relative clause is separated from the rest of the sentence by commas. If you take away the non-defining clause the basic meaning of the sentence remains intact.
For example:

• My dog chased the postman.

Adjective clauses answer questions like "which?" or "what kind of?"

Summary

An adjective clause functions as an adjective (modifies a noun or pronoun); an adverb clause functions as an adverb (describes a verb, adjective or other adverb); a noun clause is used as a noun (subject of a verb, direct object, indirect object, predicate nominative or object of the preposition).
!Note - The difference between a clause and a phrase is that a phrase does not contain a finite verb.

Relative Clauses

A relative clause follows the noun it modifies. It is generally indicated by a relative pronoun at the start of the clause, although sometimes you can tell simply by word order. The choice of relative pronoun, or choice to omit one, can be affected by the following:-

Human or Non-human?

We make a distinction between an antecedent that is a human — who(m) — and an antecedent which is a non-human — which.
Who(m) is used when the antecedent is a person.
That is used to refer to either a person or thing.
Which is used to refer to anything exept a person.

• I met a man and a woman yesterday. The woman, who had long blonde hair, was very pretty.
• The man she was with, was the man that / who won the race.
• The race was the one that I lost.
• The man, to whom the winnings were given, was with the woman who was very pretty.
  

!Note - Whom is not used much in spoken English.

Restrictive or Non-restrictive?

Restrictive relative clauses are sometimes called defining relative clauses, or identifying relative clauses. Similarly, non-restrictive relative clauses are called non-defining or non-identifying relative clauses.
In English a non-restrictive relative clause is preceded by a pause in speech or a comma in writing, unlike a restrictive clause.
For example:-

The builder, who erects very fine houses, will make a large profit.
This example, with commas, contains a non-restrictive relative clause. It refers to a specific builder, and assumes we know which builder is intended. It tells us firstly about his houses, then about his profits.

The builder who erects very fine houses will make a large profit.
This second example uses a restrictive relative clause. Without the commas, the sentence states that any builder who builds such houses will make a profit.

Restrictive		Non-restrictive
Human	Nonhuman	Human	Nonhuman
Subject	who, that	which, that	who	which
Object	who, whom, that, Ø	which, that, Ø	who, whom	which
After preposition	whom	which	whom	which
Possessive	whose, of whom	whose, of which	whose, of whom	whose, of which