‘The exercise which was really useful’, ‘The website that helped me’ - R elative Clauses

Which, what, who, that, whom? Relative clauses are very commonly used both in everyday English and academic language to link ideas within a sentence. 
On the following pages you can find general information on how to use relative pronouns and exercises to practise.

The Owl at Purdue University offers a detailed explanation of relative clauses with a little table of the words you can use to introduce them. 
http://owl.english.purdue.edu/owl/resource/645/01/

Another general introduction to relative clauses followed by a short exercise.
http://elc.polyu.edu.hk/cill/eap/2004/u6/relativeclauses.htm

In this exercise the answers are accompanied by explanations. 
http://depts.gallaudet.edu/ESL/relclauses.htm

Quizzes to help you practise:
http://www.english-test.net/esl/learn/english/grammar/ei039/esl-test.php
http://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?07

Reduced Relative Clauses
In some sentences the relative pronouns can be left out, for example ‘Improving English language skills is important for students who are studying in the UK’ can become ‘Improving English language skills is important for students studying in the UK’.

This pages has examples and a video of a teacher explaining shortened relative clauses.
http://international.ouc.bc.ca/chalkntalk/reducedclauses/

For those who prefer a written explanation.
http://www.eslgold.com/grammar/reduced_relative_clauses.html

Incomplete Sentences

Another common problem is related to sentence structure. Many students produce incomplete sentences, such as * Students who study a lot.* 
The following page explains why such sentences do not work in English. Open the page and scroll down to the last section (Sentence Structure).
http://rwc.hunter.cuny.edu/reading-writing/on-line/relative_clauses.html

Relative Clauses and Commas

The use of commas for relative clauses depends on the kind of clause you use. If you convey necessary information without which the sentence would not make sense, it is a defining relative clause and you do not need to use commas.

For example:

Shall we meet at 10.30am?
Can we make it a bit earlier? The man who helps my mother with the household is coming at 11am.

It is unlikely that the first speaker knows which man is coming, so the information “who helps my mother in the household” is necessary to understand who the man is.

If the relative clause gives additional information, it is a non-defining relative clause and you need to use commas around it.

My father, who is an engineer, encouraged me to go to university.

Here the information that your father is an engineer is not necessary to know which father the speaker means, it merely provides additional information.

Try this test to see whether you know the difference between the two:
http://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?04