Make the reference to the noun clear. For example,
The cat was playing with the dog and it seemed to be having fun.
Was the cat or the dog having fun? The sentence might be better written without the pronoun. For example,
The cat was playing with the dog and seemed to be having fun.
Many people think that they allow too much nudity on television.
Who are they? It might be better to use the noun. For example,
Many people think that producers allow too much nudity on television.
Click here for a list of pronouns.
It is quite easy to use the wrong relative pronoun, especially where that and which are involved. Relative pronouns introduce relative clauses. These clauses can be either defining or non-defining. Defining clauses add information that is essential to the understanding of the main clause. For example,
The party that the police were called to was shutdown.
Non-defining clauses add information that is non-essential to understanding. For example,
The party, which most people enjoyed, had gone on all night.
Only use that with clauses that add essential information.