A is used before the word at the beginning of a consonant (not a consonant, but a phonetic symbol);
An is used before words that start with vowels (not vowels, but phonetic symbols).
Such as: a book, an apple, an hour? Answer? University? A European country.
Usage of indefinite articles
1, used before singular countable nouns to indicate "one".
There is an apple on the table. There is an apple on the table.
2. Generally refers to people or things.
Answer? Students must study hard. Students must study hard.
3. Used before ordinal number, equivalent to another.
There is a third boy near the shop. There is a third boy near the shop.
4. Indefinite articles are used in some fixed phrases.
Really? Answer? Try it?
There are four expressions for the general citation of articles in extended materials:?
1, uncountable nouns without articles indicate general reference?
When an uncountable noun indicates a general reference, the definite article the is not added before it. If it is added, it indicates a specific reference. For example:?
Salt is a substance with strong taste, which is white powder or crystal.
Salt is a white powder or crystalline substance with a strong taste.
2. Does the plural noun refer to a general reference without an article?
When expressing general reference, plural nouns cannot be preceded by the definite article the. If you add an article, it means a specific reference, such as:
Pumas are big cats from America. ?
Jaguar is a big cat that lives in America.
3. Can singular nouns with definite articles express general reference?
When the definite article the is used with singular nouns, it can indicate the category represented by general reference and typical samples. For example:
Why should the government introduce gender budget?
Why did the government introduce gender budget?
4. The indefinite article a/an plus singular nouns can indicate general reference.
The indefinite article a/an plus singular nouns can generally refer to any representative member of a certain kind of thing. For example:
Puma is a member of the cat family. ?
Jaguar is a wild cat.