1. Applications
1.1. 1. It is used to talk about something in general
1.1.1. Examples
1.1.1.1. He has a computer
1.1.1.2. I want an orange
1.2. 2. It can also be used instead of "1"
1.2.1. Examples
1.2.1.1. I want an apple and two oranges
1.2.1.2. There are a hundred students in the school
1.3. 3. To talk about someone or something
1.3.1. Examples
1.3.1.1. Carla lives in a nice big house
1.3.1.2. My uncle is an engineer
1.4. 4. To describe
1.4.1. Examples
1.4.1.1. She is a doctor
1.4.1.2. That is an elephant
2. Rules
2.1. 1. The article a is used for names and nouns that begin with consonants
2.1.1. Examples
2.1.1.1. a ball
2.1.1.2. a cup of coffee
2.2. 2. the article An is used for names and nouns that begin with a vowel
2.2.1. Examples
2.2.1.1. an umbrela
2.2.1.2. an ice cream
2.3. 3. Do not use a / an with names or nouns that are written in plural or are countless, for a / an means "one"
2.3.1. Examples
2.3.1.1. My grand parents are teachers (not...a teachers)
2.3.1.2. Would you like some bread? (not...a bread)
2.4. 4. Do not use a/an with this single adjective without a noun
2.4.1. Examples
2.4.1.1. Carla is a very good architect
2.4.1.2. Carla is very good (Not Carla is a very good)
3. Exceptions
3.1. 1. If using a word with an 'h' move, as 'honourable' or 'honest' then you have to use 'an'
3.1.1. Examples
3.1.1.1. an honourable man
3.1.1.2. an honest mistake
3.2. 2. When the 'u' has the same sound as the 'and' in 'you', or the 'o' has the same sound as the 'w' in "won" then you need to use 'a'. example
3.2.1. Examples
3.2.1.1. a one-legged man
3.2.1.2. a European trip