Sentences that use good grammar and syntax are essential for clear writing. Here, you will find resources on some of the most common sentence errors and advice on how to make your meaning clear at a sentence level.
The clauses you were asking about are adjective clauses, and they are divided into two types--but there are a number of different names for these two types of adjective clauses:
Restrictive clauses (also called "essential", "defining", or "identifying" clauses). These clauses are not set off by a comma and the information provided is needed (essential) to identify the noun that they qualify;
e.g. My neighbour who lives in the two-storey house is a good friend of mine.
Non-restrictive clauses (also called "non-essential", "non-defining", or "non-identifying" clauses). These clauses are set off by a comma and the information provided is just additional information; it is not needed to identify the noun that they qualify;
e.g. Mrs. Smith, who is my next-door neighbour, is a good friend of mine.
There is a further difference when using clauses requiring "that" or "which". Use "that" in restrictive clauses and "which" in non-restrictive clauses.
e.g. The two-storey house that belongs to the Smiths is next door to us; your friend's bungalow, which is a much smaller house, is on the other side of ours.
Recognition of Sentence Parts, a module of the online English Grammar Handbook.
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Updated September 10 2014 by Student & Academic Services