There are three possibilities with articles: a/an (the indefinite article), the (the definite article) and nothing (the zero article):

We stopped near a lion and a tiger. The lion noticed us but the tiger didn’t.
Lions are fascinating animals.

The indefinite article, a/an is used:
- before a singular, countable noun when it is used for the first time, as in the example above: We stopped near a lion and a tiger.
Before plural countable nouns used for the first time we use some and before uncountable nouns either some or no article:
We stopped near some lions.
I need some information.

The definite article, the, is used before singular and plural countable nouns and uncountable nouns when it is clear what is being referred to, either because
- it has been referred to directly before, as in the example above:
The lion noticed us but the tiger didn’t.
- it has already been referred to indirectly:
I visited a zoo recently, and really enjoyed watching the animals.
The zoo implies that there were animals there.
- it is clear from the situation
I took the children to the zoo yesterday.
Here the speaker assumes that the listener knows which children I’m referring to (probably my own) and which zoo (probably the only one where I live).
- it is unique, or generally talked about as though it is unique
The Earth, the sun

No article is used
- with uncountable nouns
Zoology is a fairly popular subject.
- when something is referred to in general
Lions are fascinating animals.


These activities practise this Grammar: