(Contributed by Ben)
The singular ‘they’ is a very common feature of conversation, but I rarely hear it in classes. It’s also slightly controversial. Some native speakers don’t like it, but most use it anyway.
Let me explain. The ‘singular they’ is when you use ‘they’ to refer to one person (even though ‘they’ normally refers to two or more people). This is actually very common. If somebody knocks on your door, you might say ‘I’ll let them in’ because you don’t know if the person is male or female until you open the door. You could say ‘I’ll let him or her in’ but that takes longer, and sounds awkward.
Let me give another example. ‘When a person wants to eat a meal, they must cook it first’. ‘They’, in this example, is the imaginary ‘person’. Alternatively, you could say ‘When a person wants to eat a meal, he or she must cook it first’. Some people also suggest saying ‘he must cook it first’, and use ‘he’ to mean ‘he or she’, but this practice is quite old-fashioned, and excludes women – it only became popular 150 years ago. Yet another option: you could make your hypothetical person plural and say ‘When people want to cook meals, they must cook them first’.
So you see, there are several options when you’re referring to a person without knowing their gender: ‘he’ (using male as the default), ‘he or she’ (giving two options), ‘she or he’ (a less common version of this – I have seen it used in some academic papers) or the singular ‘they’.
Some people don’t like the singular they, because it’s used with plural verbs. For example, when you hear a knock at the door, you could let the person in ‘to see who they are’. Even though ‘they’ is used to mean one person in this example, you still need to use ‘are’ (as if it was plural). This can be confusing. (It makes a particular difference in some political and religious texts). Still, the singular ‘they’ has been around for hundreds of years. Shakespeare, Jane Austen, any many other great authors used it casually.