Uses of Indefinite and Definite Article

A or An:  Originate as same as "one", is mainly used before countable singular noun.

1.1 Means one, as a number :

Examples:

• Rome wasn't built a day.
• She is a friend of mine.
• I take that bus 2 times a day.
• The ship goes by 30 km an hour.
• A car is waiting at the door.

1.2 Means someone, not specified to certain thing or person :

Examples:

• He was born in a small city.
• They are waiting for a visitor.
• An interesting story always makes kids laugh.
• She is a Canadian writer.

1.3 Means one type or class of something :

Examples:

• A child needs love.
• An owl can see in the dark.
• An honest man usually is a fair man.

The:  Originate as same as "this" or "that", weaker than "this" or "that" but can be used on both singular and plural objects; it can be used with adjective too.

2.1 Means specific thing or things before singular or plural Individual noun :

Examples:

• Where are the other girls?
• Put the phone on my desk.
• Show the quests our meeting room.
• Did you go to the opening ceremony?

Other nouns, not individual nouns, should be led by definite article too, only if they represents specific thing. For example,

• Pass me the salt. (Mass Nouns)
• Take the medicine now. (Mass Nouns)
• I like the children's performance. (Abstract Nouns)

When we mention something second time in context, use definite article to specify it. For example,

• I buy a computer and a printer, but I dislike the printer.
• I enter a house in the street and see the rooms are very clean.

2.2 Means a specific type of something before singular noun :

Examples:

• The whale is in danger of extincting.
• The 18 century is the golden age of the novel.
• The plane has made the earth smaller.
• The computer is changing our life.

2.3 Means the whole of an ethnic, class, or family before noun :

Examples:

• The Thompsons arrived at 8 o'clock.
• The Japanese people like sushi.
• They represent the interests of the middle class.

2.4 Used before noun to express unique object :

Examples:

• Do you know how big the universe is?
• The sun lights the earth.
• The Mike Way is bright in the sky.
• The moon is in the middle of the air.
• They arrived the North Pole.
• His shuttle is leaving the outer space.

2.5 Used before adjective to express a type of people :

Examples:

• He shouldn't be among the unemployed.
• These seats are for disabled.
• Either the rich or the poor have same right to vote.

2.6 Used before adjective to form a noun phrase :

Examples:

• Our goal is to discover the good, the beautiful, and the true.
• She admires the mystical.
• I like to venture the unknown.