The articles are not used:

1.    Before proper, material and abstract nouns used in a general sense:

London, France, Tom, gold, rice, honesty, virtue

Paris is the capital of France. (NOT The Paris is …)
Gold is a precious metal. (NOT The gold is a …)
Honesty is the best policy.
But we say, the United States, the United Arab Emirates etc.

2.    Before a common noun used in its commonest sense:

Man is mortal.
Iron is a useful metal.

3.    In certain phrases made up of a preposition + noun:

Examples are: on foot, at school, from top to bottom, at home, in bed, by train, by car, in debt, in hand, on earth, at noon, on board, in jest, at best, at worst etc.

He is in debt. (NOT He is in the debt.)
He is at school. (NOT He is at the school.)
He spent the whole day in bed.

4.    In certain phrases consisting of a transitive verb and its object:

Give ear, set sail, take heart, send word, catch fire, take offence, leave office, leave home, leave office etc.

He took offence at my words. (BUT NOT He took the offence at my words.)
She left home in the morning. (BUT NOT She left the home in the morning.)

5.    Before the names of continents, countries, capes, cities, towns, days, months, arts, languages, sciences, some diseases etc.

January, March, physics, English, Tokyo, Beijing, Delhi, Tuesday, Friday, Asia, America etc.

December is a cold month.
English is spoken all over the world.

6.    Before common nouns when they go in pairs:

Both husband and wife received serious injuries.

7.    Before plural nouns used to denote a class:

Apples are red.
Camels are useful animals.
Warm clothes are necessary in cold climates.

8.    Before the nouns following kind of:

What kind of flower is it? (NOT What kind of a flower is it?)

9.    Before meal-time

He was at dinner.
Breakfast was served at night. (BUT NOT The breakfast was served at night.)

Some special points

Compare the pairs of sentences given below:

I have a black and white cow. (Only one cow)
I have a black and a white cow. (Two cows)
The secretary and accountant is present. (Here the nouns secretary and accountant refer to the same person.)
The secretary and the accountant were present. (Here the repetition of the articles implies that the secretary and the accountant are different persons.)