The Indefinite Article A/AN

The indefinte article a/an is the same for all genders: a girl, a cat, an eraser, an egg.

The indefinte article has no plural form: a girl - girls, a cat - cats, an egg, eggs.

We use an if the following word starts with a vowel sound:
- an egg, an envelope, an apple.

We use a if the following word starts with a consonant sound:
- a cat, a dog, a door, a girl

We must use the indefinite article a/an before phrases of time and measurements (per week/weekly):

- We have Spanish classes 3 times a week.
- I go on holiday once a year.
- This Ferrari can do 350 kilometres an hour.
- Oranges are $1 a kilo.

Before phrases of jobs:
- George is an engineer.
- I'm a teacher.

Before phrases of nationality:
- Elton John is an English singer
- Antonio Banderas is a Spanish actor

Half/Quite:
- Stella needs half a kilo of sugar
- This is quite a good song

So far, so good. Further on, I'll explain about the article the.