A relative clause is a subordinate clause that begins with a question word (e.g. who, which, where) or the word that. You can use it to modify a noun or pronoun (i.e. to identify or give more information about it).

• Students who can develop independent learning skills often achieve good academic results.
• There is a new book that investigates the controversy over political reforms in Hong Kong.
• A university is a place where people pursue advanced knowledge in specific academic disciplines.
• The lecture theatre in which the inauguration ceremony will be held is now being cleaned.

Words like who, that and when are often referred to as relative pronouns when they are used to introduce relative clauses. You use:

• who for people, which for things, and that for both people and things.
• whom as the object of a relative clause (in more formal English), though it is increasingly common to replace it with who.
• whose to indicate possession, as a determiner before nouns.

• What’s the name of the person who/that first landed on the moon?
• This is Dr. Perkins, whom we met at a conference in Canada last year.
• All students whose registration numbers begin with 374 should immediately go to the library for a tour.

There are two types of relative clause: defining and non-defining. You use a defining (or restrictive) relative clause to ‘identify’ or ‘restrict the reference of’ a noun. You do not separate it from the rest of the sentence by commas (in text) or pauses (in speech).

• The student who achieves the highest GPA score in this department will be awarded a prize of $20,000.
• Computer games that involve fighting and shooting apparently have a negative effect on young people.

You use a non-defining (or non-restrictive) relative clause to supply additional information about the noun, whose identity or reference is already established. You can also use it to comment on the whole situation described in a main clause.

• Albert Einstein, who put forward the theory of relativity, is considered by many as the most intelligent person in human history.
• The ELC, which provides language support to PolyU students, is located in the AG wing.

You should not use the relative pronoun that in non-defining relative clauses.

You can sometimes omit the relative pronoun in a defining relative clause to create a more concise style. You cannot do this in a non-defining clause.

• Half of the training sessions (that are) arranged for the athletes have been cancelled due to bad weather.
• The designer (who/whom) you saw at the party last night is Giorgio Armani!
• * 'The ELC, provides language support to PolyU students, is located in A core.' is not correct.