You’re in a rush and you just can’t recall whether you need to use that or which in a sentence or, perhaps, your dilemma is choosing between who and whom. Maybe the choice you’re facing is between all four.
Then fear not. There are simple reminders you can use.
Firstly, sentences that have restrictive clauses (ones that could not be removed from the sentence or it would no longer make sense) require that and non-restrictive clauses (ones that can be discarded without changing the meaning of the sentence) require which.
The chaos that follows a cyclone has to be seen to be believed.
Apples, which are grown widely in Australia, are a popular fruit.
(TIP: restrictive clauses are properly surrounded by commas)
Secondly, the choice between using who and whom confuses most writers at some time. A simple way to remember the difference is to use who when the pronoun (a person) is the subject of a verb and whom when referring to the (direct or indirect) object of a verb or the object of a preposition.
Voters who were dissatisfied with the size of the ballot paper can complain to the electoral commission.
Mention was made of every student who had been elected as a prefect.
The candidate whom the voters elected must now deliver on his promises.
This election’s result was so tight, politicians can no longer ignore those voters with whom they disagree.
Finally, remember to use who or whom when referring to people and to use that or which when referring to things, animals or inanimate entities.