explanation

EXPLANATION
1. a/an is called the indefinite article, while the is called the definite article. A is use before a consonant (sound, not letter) and an before a vowel (sound, not letter). The is used before any sound: a consonant or a vowel.
For example:
A man, a university; but an apple, an hour
An apple, an accident; but a sweet apple, a terrible accident.
The man, the university, the apple, the hour
2. a/an is used with singular common nouns which are not specifically identified. A common noun, also called a generic noun, represents a class or group of objects or thing.
For example:
Bob ate an apple.
The meaning of an here is similar to one. The speaker is not talking about this apple or that apple. He just eats one apple. The listener does not know which specific apple was eaten.
A is not used with plural nouns and uncountable nouns, such as water, milk, bread. Some may often be used instead.
For example:
Bob ate some apples. (also three, a lot of, a few apples, etc.)
John drank some milk. (Also a lot of, a little, a glass of milk, etc.)
3. a/an is also used for singular common nouns to make generalizations and it represents a whole class of objects.
For example:
An apple is not always sweet.
In this sentence, the speaker is talking about any apple or all apples in general. With the same meaning, we may also say
Apples are not always sweet (a/an is not used for plural nouns)
In the same grammatical context, i.e. to make generalizations, we do not use a/an for uncountable nouns.
For example:
Water is vital in our life.
Milk is expensive nowadays
4. The definite article the is used with nouns representing specific or familiar objects, persons, or incidents. The meaning of the here is similar to this, that, these or those.
For example:
There is an apple in the kitchen. The apple is sweet.
The milk in the refrigerator is still fresh.
The speaker uses the in the second sentence because the word apple has become specific or familiar to both the speaker and the listener. It has been mentioned in the first sentence, so the listener knows which apple the speaker is being talking about.
The speaker uses the for kitchen and refrigerator because both the speaker and the listener understand which kitchen and refrigerator the speaker is talking about. The two words are specific and familiar to them because there are only one kitchen and one refrigerator in the house.
The is used for milk because the word milk is made specific and familiar by the phrase in the refrigerator. Both the speaker and the listener understand which milk the speaker is talking about.
5. Almost all proper names do not use any article. There are only a few exceptions, such as the United States, the Philippines.
For example:
Obama is the new president of the United States.
Japan is the richest country in Asia.
Obama and Japan do not use any article because they are proper names. The United States is also a proper noun but it uses the definite article. It is exceptional.

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