A is the indefinite article. It is used with single countable nouns.
Use a before consonant sounds. Use an before vowel sounds.
The is the definite article. It is used when there is only one of something.
This includes superlative adjectives.
It is used when it is clear which thing the speaker refers to.
It is used when something has already been identified.
We don't use the article the when we talk about things in general.
The zero article is used with indefinite plurals and uncountable nouns. We use it when we are talking about these things in general.
The article the is used with specific plurals and uncountable nouns.
Here are more examples to compare:
3. Correct or incorrect?
Are these sentences correct or incorrect?
For more information about articles see the Grammar Reference.