2. A noun is definite when the speaker and listener both know which particular person, place, or thing is being talked about.

3. THE is used in measuring expressions beginning with by.
For example :
Incorrect : Do you sell eggs by kilo or by dozen?
Correct : Do you sell eggs by the kilo or by the dozen? 

4. Definite article is not usually use before people's names.
Incorrect: The Abraham Lincoln was the 16th president of the United States.
Correct:  Abraham Lincoln was the 16th president of the United States.


Example:

Incorrect use for definite:
1. Why are you reading the my book?
2. The Paris is a big city.
3. The iron is useful metal.
4. The gold is yellow.
5. I live in the Mumbai. 

2. A noun is often indefinite the first time a speaker mentions it. It is usually definite after the first mention.

The choice between A and AN is determined by sound and not spelling.

A is used before words beginning with a consonant sound. Examples are: a boy, a tree, a ball, a flower, a horse, a hole, a European and a university.

AN is used before words beginning with a vowel sound. Examples are: an elephant, an egg, an orange, an umbrella, an hour, an heir etc.

Some is used for plural nouns. For example, some students, some rice, etc.

Note that some native speakers use AN before words beginning with h if the first syllable is not stressed.

Incorrect use for indefinite:
1. My mother is a honest woman.
2. She is an United States senator.
3. Please give me an ice.
4. Please give me a water.

3. Zero article are generally used with means of transport ("by plane") and common expressions of time and place ("at midnight," "in jail").

Examples:-

Incorrect use of zero article:
1. Let's go out for a dinner.
2. The creativity is a valuable quality in children.
3. I studied the French in high school for four years.
4. I like to play the baseball.
5. My sister was always good at the math.

The indefinite article (A/AN) is used before singular countable nouns. It cannot be used before plural nouns.

Use zero article (no article) with non-specific plural count nouns, non-specific non-count nouns, names of people, names of most countries, and habitual locations.

 

Use AN before nouns (or adjectives) that start with a vowel sound.

Remember that you're listening here. This isn't about the letter c or d being a consonant or the letter o being a vowel; it's about the sound they make

THE is used in front of singular or plural nouns and adjectives to refer to something with which both the speaker/writer and listener/reader are familiar.