A / an are determiners. Determiners are words we use before a noun to show whether the noun is specific or general, singular or plural, etc.

A/an

We use a or an with singular countable nouns only. A and an are indefinite articles. We use them to talk about one of something when we assume that the listener / reader doesn't know which specific thing:

• A car drove past. (= we don't know exactly which car)

The

We use the with countable nouns (singular or plural) and uncountable nouns:

• A man is coming round to fix the television.
• Let's sit on the grass over there.

The is the definite article. We use it to talk about a specific example of something we think is known to both ourselves and the listener / reader:

• The cars were parked illegally in the city centre. (= we know which cars and which city)

THE WITH NOUNS THAT ARE ALWAYS SINGULAR

We nearly always use the with some singular nouns because we consider there is only one in existence:

• the sun the moon the Earth the air
• the ozone layer the past the future the countryside
• the EU the UN the seaside the world
• the Vietnam War the presidency the Government

This category also includes superlatives because there is usually only one thing or group that is superlative:

• He's the best accountant in town.
• It's one of the noisiest bars in town.

Logic is not always a reliable guide. We talk about the atmosphere and the environment. But we usually think of nature in a general sense and so omit the. Although we talk about the universe, we consider space as infinite and we use it without the:

• The oldest man in the space was John Glenn