How do we use 'a' and 'the'? What is the difference and when do we not use them?


A/an and the are articles. The is a specific or particular article. A/an is a non-specific or non-particular article.
- "Let's read a book!"
- "Let's read the book called On the road by Jack Kerouac. "


The is also used for a specific article in a group of things. 
- "Let's see a movie"
- "Let's see the movie, which won the most oscars last year?"


There's a difference between a and an:
A is used if the noun it refers to begins with a consonant:
- a boy, a car
An is used if the noun it refers to begins with vowel:
- an apple, an egg
A is used if the singular noun beginning with a consonant sound: 
- a user, a university, a unicycle
An is used if the noun starts with a silent "h":
- an hour
A is used if the noun starts with a pronounced "h":
- a horse


By Coni