Referring to a particular thing using the definite article or indefinite article

Basically, you use the when you think your listener will be able to identify the thing you are referring to, whether or not it has been explicitly referred to before. Otherwise, you use a or an (if you are referring to one thing using a count noun). So when you use the you imply to your listener that you are referring to an identifiable thing, person, or group.

Why didn't they all follow him into the living-room?

WARNING: One thing that is often said is that the first time you mention something you use a or an, and the second time you use the. This is only occasionally true; people do not usually use the and the same noun when referring back to something they mentioned before. There are three sorts of information which listeners and readers can use to work out why the definite article has been used and to identify what items are being talked about. These are:

1) what has been said earlier in a conversation or text. This process is sometimes called referring back or anaphora.

But it seemed Mrs Colombo owned a dog which her youngest son adored. The landlord had received complaints about the dog barking at night.

2) the context or situation in which you are speaking or writing.

Did you enjoy the concert?

Obviously the situation is much clearer when you are speaking with someone face-to-face, and so this use is particularly important in speech.

3) the language that you use with the noun, as part of the noun group.

We look at it in a bit more detail at the end of the chapter.

Don't forget that there are other determiners which give more precise information than the and which have to be used sometimes in cases where you might expect something to be identifiable simply with the. For example, you generally use a possessive determiner when referring to part of someone's body, someone's relatives, and someone's personal possessions; you would say 'Sarah's hurt her arm' or 'John has lost his wallet', not 'Sarah's hurt the arm' or 'John's lost the wallet', And you generally use the determiners 'this' and 'that' when drawing attention to something; for example, you would say 'I hate living in this flat', not 'I hate living in the flat'.