a = indefinite article + words starting with consonant.
- She has a flat in the centre.
- Can I have a glass of water, please?
- He is a doctor.
an = indefinite article + words starting with vowel.
- She always wears an old dress.
- Can I have an ice cream, please?
- I am reading an interesting book.
The indefinite article a or an is used with singular nouns to refer to a thing for the first time.
The = definite article.
- The flat is very nice.
- The ice cream is disgusting!
- The book is on my desk.
The definite article the is used with singular and plural nouns when both the speaker and the listener know the thing already.