An adjectival clause acts as an adjective in a sentence, that is, it modifies a noun or a pronoun.
The bushman, who knew the forest well, told us about the hidden cave. (Who knew the forest well is an adjectival clause that modifies the noun bushman).
The bushman told us a legend that involved the cave. (That involved the cave is an adjectival clause that modifies the noun legend).
An adjective clause usually comes immediately after the noun it modifies.
People still search for the treasure that the pirate hid.
As can be seen from the above examples, adjectival clauses, like adjectives, modify nouns or pronouns answering questions like which? or what kind of?
The red coat the coat which I bought yesterday
Like the adjective red the adjectival clause which I bought yesterday modifies the noun coat.
Note than an adjectival clause usually comes after what it modifies while an adjective comes before.
Besides use of subordinating conjunctions, adjectival clauses can be introduced by relative pronouns.
Relative pronouns are the words who, whom, whose, that and which.
These words relate the subordinate clauses to the word it modifies in the main clause.
The books that people read were mainly religious.
Some fire-fighters never meet the people whom they save.
The meat which they ate was rotten.
In the last sentence, the relative clause (called so because it is introduced by the relative pronoun which) which they ate modifies the noun meat and answers the question which meat? More Examples:
They are searching for the one who borrowed the book.
The relative clause who borrowed the book modifies the pronoun one and answers the question which one?
Besides relating the adjectival clause to a noun or pronoun in the main clause, a relative pronoun may also act as the subject, object, predicate pronoun, or object of a preposition in the clause.
Subject: This is the forest that has a secret cave. (That is the subject of has)
Object: The map, which you saw, guides the way. (Which is the object of saw)
Object of a preposition: The map leads to the cave of which the bushman spoke. (which is the object of the preposition of)
In informal writing or speech, you may leave out the relative pronoun when it is not the subject of the adjectival clause, but you should usually include the relative pronoun in formal academic writing.
Formal: The books that people read were mainly religious.
Informal: The books people read were mainly religious.
Formal: The map which you saw guides the way.
Informal: The map you saw guides the way.
But never omit the relative pronoun if it is in the clause.
Correct: This is the forest that has a secret cave.
Incorrect: This is the forest has a secret cave.
Commas are put around adjectival clauses only if they merely add additional information to a sentence.
The map, which you saw, shows the way.
This adjective clause can be left out without affecting the grammatical structure of the sentence.
It is merely adding information to the sentence by telling us which map?
The map shows the way.