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)