relative Clause

Adjectival (or relative) clauses can function only as postmodifiers of a nominal head. This nominal head is called antecedent, and the relative pronoun that introduces the relative clause is replacing the nominal head. Relative clauses may appear both in the subject and in the predicate of a sentence.
[in the examples below, the clauses appear between parentheses]
"The girl (who lent me the books) is not here today."
The girl who lent me the books --> subject
is not here today --> predicate
girl --> head of the subject (noun)
who lent me the books --> adjectival clause, it postmodifies 'girl'. 'Who', the relative pronoun that introduces the clause, replaces 'girl'. We're not talking about just any girl but about the one who lent me the books.
Relative pronouns have a double function in the sentence:
a. they introduce the relative clause
b. they have a function within the clause itself.
[In the example: who --> subject of the clause
lent me the books --> predicate of the clause]
Relative clauses are generally introduced by relative pronouns: who, whom, whose, which, that.
1. "This is the house (that I want to buy)."
2. "I know the man (whose wife works in the library)."

3. "The book (which I told you about) is out of print."
4. "The house (in which Jack lives) has a large garden."
5. "Mr. Brown, (whom you met at the club), is giving a party tonight."
6. "Bernard Shaw, (who wrote St. Joan), died in 1950."
7. "My car, (which broke down last night), is in the garage."
There are two types of relative clauses: "defining" or "restrictive", and "non-defining" or "non-restrictive".
- Sentences 1, 2, 3 and 4 contain examples of "restrictive" relative clauses. In those sentences the clause is a necessary part of the idea; if it is left out, the sentence may not make complete sence. These clauses define the antecedent (noun), they indicate, for example, which house out of all the possible houses, what man out of several possible men.
- The relative clauses in sentences 5, 6 and 7 are "non-restrictive". This means that the relative clause could be omitted and the rest of the sentence would still make sense. If we said "Bernard Shaw died in 1950", the sentence would still have meaning. The relative clauses in these sentences provide additional information that could be avoided. Non-restrictive relative clauses are differentiated from restrictive relative clauses by the use of a comma at the beginning and end of the clause (a comma in writing, and a pause in speaking).
'as' can also introduce a relative clause, after 'same' and 'such':
"I'll be surprised if he does the same way (as I do)."
"I've never heard such stories (as she tells)."
Relative clauses may even be introduced by 'where', 'when', 'why' if these words have the meaning 'in which', 'at which', 'by which', etc. (Be careful with these; sometimes these clauses are confused with adverbial clauses.)
"The place (where we met) was a lovely park."
"I remember the day (when she got married)."
"I don't know the reason (why you're so angry)."
In restrictive relative clauses, the relative pronoun is often omitted (mainly in spoken English) when it is the object of the clause.
In the sentence "The book (that I want) is on the table", the relative clause is "that I want". "I" is the subject of the clause, "that" and "want" form the predicate. Within the predicate, "want" is the verb and "that" (which is replacing the book) is the direct object. We could rephrase the clause as "I want that" --> 'that' meaning the books.
This is the case when you can omit the relative pronoun. you can say either "The books (that I want) are on the table" or "The books (I want) are on the table".
Even if you omit the relative pronoun, you will still have a relative clause. In this type of clause, we speak of "zero pronoun".
RELATIVE CLAUSE-in other words- : A relative clause is a subordinate clause that modifies a noun. For example, the noun phrase the man who wasn't there contains the noun man, which is modified by the relative clause who wasn't there. In many languages, relative clauses are introduced by a special class of pronouns called relative pronouns; in the previous example, who is a relative pronoun. In other languages, relative clauses may be marked in different ways: they may be introduced by a special class of conjunctions called relativizers; the main verb of the relative clause may appear in a special morphological variant; or a relative clause may be indicated by word order alone. In some languages, more than one of these mechanisms may be possible. A clause is a group of words consisting of a subject (often just a single noun) and a predicate (sometimes just a single verb). ... A noun, or noun substantive, is a part of speech which can co-occur with (in)definite articles and attributive adjectives, and function as the head of a noun phrase. ... In linguistics, a noun phrase is a phrase whose Head is a noun. ... In linguistics and grammar, a pronoun is a pro-form that substitutes for a noun phrase. ... A relative pronoun is a pronoun that marks a relative clause within a larger sentence. ... In linguistics, a relativizer is a conjunction used to indicate a relative clause. ...
The antecedent of the relative clause (that is, the noun that is modified by it) can in theory be the subject of the main clause, or its object, or any other verb argument. However, many languages do not have the possibility, or a straightforward syntactic pattern, to relativise arguments other than the core ones (subject and direct object). A syntactic verb argument, in linguistics, is a phrase that appears in a relationship with the verb in a proposition. ...
In English, a relative clause follows the noun it modifies. It is generally indicated by a relative pronoun at the start of the clause, although sometimes simply by word order. The choice of relative pronoun, or choice to omit one, can be affected by whether the clause modifies a human or non-human noun, by whether the clause is restrictive or not, and by the role (subject, direct object, or the like) of the relative pronoun in the relative clause. In English, as in some other languages (such as French; see below), non-restrictive relative clauses are set off with commas, but restrictive ones are not: This article is focused mainly on usage of English relative clauses. ... In semantics, a modifier is said to be restrictive if it restricts the reference of its head. ...
I met a man and a woman yesterday. The woman, who had a thick French accent, was very pretty.
I met two women yesterday, one with a thick French accent and one with a mild German one. The woman who had a thick French accent was very pretty.

As regards relative clauses, English has two particularities that are unique among the Germanic languages:
In other Germanic languages, if a relative pronoun is the object of a preposition in the relative clause, then the preposition always appears at the start of the clause, before the relative pronoun. In English, the preposition will often appear where it would appear if the clause were an independent clause — in other words, the relative pronoun "strands" it when it moves to the start of the clause. It used to be common to regard this as a grammatical error (see: Linguistic prescription) but in fact it has been a standard feature of the language since Middle English times.

In other Germanic languages, a relative pronoun is always necessary. In English, however, it may be suppressed in a restrictive clause, provided it would not serve as the subject of the main verb. When this is done, if the relative clause is the object of a preposition in the relative clause, then said preposition is always "stranded" in the manner described above; it is never moved to the start of the clause.