Relative clauses, also known as adjective or attributive clauses, are a type of complex sentence in English grammar.
The two main types are defining and non-defining relative clauses.
Relative clause fragments are when the independent clause is missing in your sentence.
Relative clauses describe or provide information about someone or something (a noun or noun phrase) that has usually already been specified:
Relative clauses are also a way in which clauses can be combined so that we do not repeat ourselves:
Relative clauses begin with one of these relative pronouns:
Or one of these relative adverbs:
These words though, are not just used for relative clauses, so you can't assume if you see one it is a relative clause.
It is the sentence position and the context that will tell us whether it is a relative clause or not.
Relative clauses follow whatever it is that they are qualifying.
They commonly qualify or give more information about a noun. This is why they are also known as adjective clauses.
An adjective describes or clarifies a noun, and in a similar way, a relative or adjective clause gives more information about or defines a noun.
In this case they will come directly after the noun:
If they are qualifying a whole clause, then they come directly after that clause:
You'll have noted in the above examples that the relative clause comes after the main clause.
However, they can also spit up the main clause. This is if they are qualifying a noun which is the subject of the sentence, in which case they are embedded within the main clause:
When we make sentences with relative clauses, we are joining two independent clauses, which means there will be a subject and verb in each clause:
The verb in the relative clause must also agree with the noun that it is modifying. So in the above example, which is modifying car, and car is a singular noun. So it takes a singular verb, is.
If it was a plural noun, it would take a plural verb:
Relative clauses are either defining or non-defining.
Defining means that they are essential to the sentence and cannot be omitted. In this case, commas are not used:
If we leave 'who stole my bag' out of the sentence, we don't know what 'she' did. It is thus an essential clause.
In these types of clause, that can also replace the relative pronoun:
Non-defining relative clauses add extra information, or non-essential information, to the sentence:
The main information about the zoo is that it is one of the best in the country. 'Which was built seven years ago' is extra information and the sentence makes sense if we leave it out.
Commas are also used in the second example to separate off this non-essential information. Also, that cannot be used.
You can learn more about the differences here.