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.