A and an are two forms of indefinite articles in English that are used to modify a singular noun to indicate that the noun is generic rather than specific. The main difference between them is related to the pronunciation of the noun it modifies.
When "a" is used:
When a noun begins with a consonant phoneme (not a consonant letter), use "a". For example: a book, a cat, a university. The "consonant phoneme" here refers to the phoneme that does not vibrate the vocal cords during pronunciation, such as b, d, g, k, etc.
It is important to note that some words that start with a consonant letter still use "a" if their pronunciation begins with a vowel phoneme. For example: a useful book, a university student. This is because their pronunciation begins with vowel phonemes, although they begin with consonant letters.
When "an" is used:
When a noun begins with a vowel phoneme (not a vowel letter), use "an". For example: an apple, an hour, an island. The "vowel phoneme" here refers to the phoneme in which the vocal cords vibrate during pronunciation, such as a, u, u, etc.
Similarly, some words that begin with a vowel letter use "a" if their pronunciation begins with a consonant phoneme. For example: a European country, a one-way street.
In general, the choice of a and an depends on the pronunciation of the noun being modified, not its spelling. In order to choose correctly, one needs to pay attention to whether the first phoneme of a noun is a vowel or a consonant.
In addition, the main function of the indefinite article a an is to refer to any one of a certain class of people or things, which is equivalent to "one" in Chinese. They are often used to describe an unspecific person or thing, as well as a general, first-mentioned person or thing. At the same time, the indefinite article can also be used to refer to a class of people or things in general, indicating the quantity "one", but does not emphasize the concept of quantity, and only indicates that the noun is unspecific.