Relative pronouns; There are several problems with these, not the least being that usage can vary from person to person. Notably the question of using "that" is fraught with complications and, for most users the best solution is probably to avoid it altogether and to use "which", "who" or "whom" instead.

It is useful to recognise defining and non-defining clauses because if it is defining and if it is the object you can miss out the pronoun but not in other cases.
When they are written the distinction is relatively easy because the clause is set between commas.

The man who had a hat took out a gun. (the fact that he was wearing a hat differentiated him from other men present.

(Defining) is quite different from:

The man, who had a hat, took out a gun.

(Non-defining - We know already which man we are talking about and indeed, there may have been no other men present. The information that he was wearing a hat is incidental. You can't miss out the pronoun here because the pronoun "who" refers to the subject of the sentence.)

Here is another example:

The program which I have just loaded onto my computer seems to be incompatible with Internet Explorer.
(Defining)
The program, which I have just loaded onto my computer, seems to be incompatible with Internet Explorer.
(Non-defining)

In the former, because this is an object case, you can omit the pronoun "which" and say:

The program I have just loaded onto my computer seems to be incompatible with Internet Explorer.

However, in the spoken language it is a question of emphasis and pauses. Listen to the following:

	Defining
	Non-defining

The aim of the following exercise is to distinguish between these two forms when listening.

When you have done this, go on to the second activity (link at top).