Rule 1: When a speaker speaks of a non-specific noun, then Indefinite article is used. e.g.
- A boy asked me the way to post office.
- A stray dog bit the girl.
Rule 2: With measurements and rates and numerical sense. e.g.
- Take this medicine four times a week.
- He visits me three times a month.
- She did not utter a single word.
- He ate a mango.
Rule 3: Indefinite article is used to make a general statement. e.g.
- A book is a source of knowledge.
- A phone is a very useful gadget for communication.
Rule 4: A is used before consonant. e.g.
A boy, a child, a man, etc.
Rule 5: If a noun starts with vowel sound, then ‘an’ is used before the noun. e.g.
An hour, an eagle, an eye, etc.