Relative clauses are not always the easiest of concepts, but once you have learnt the basics they should come naturally. There are some key things to remember when using relative clauses:
Is the information in the relative clause essential to the meaning of the sentence?
This will inform you whether your relative clause should be defining (essential) or non-defining (not essential). The answer to this question will affect how you punctuate the sentence and which relative pronouns you use.
Look at the following sentence: The book which I wrote last year made me famous.
The relative clause is in bold. There are no commas around the clause, so we know that this is a defining relative clause (the information is essential).
Compare this with: The book, which I wrote last year, made me famous.
Here you can see that there are commas around the relative clause. This means that the relative clause is non-defining, so the part where the writer tells us that the book was written last year is extra information.
The punctuation is easy; commas for extra information, no commas for essential information.
The key to knowing when to use a defining or non-defining relative clause is in the meaning. Look at both of the sentences below. Why is the same information essential in one and not essential in another?
My book which I wrote last year made me famous.
My book, which I wrote last year, made me famous.
Firstly, the clause describes the book. In sentence one, it is important to know that it was the book written last year, not another book. In sentence two, it is not important to know it was the book written last year. Why? Because in this sentence the book is clearly defined. In this sentence the writer has only ever written ONE book, so we don’t need essential information to describe which one.
Remember that in sentence 1 above you can use that as well as which. You cannot use that in sentence 2.
Relative clauses are a good way of developing your sentences and making your writing more academic because they give you the opportunity to add extra information.