“a” and “an” is used to refer to a non-specific or non-particular member of the group.
For example, “a computer” means any computer rather than a specific computer.
It is also used before nouns that introduce something or someone you have not mentioned before. For example,
“I read a book this morning.”
A and an are also used when talking about your profession. For example:
“I am a network engineer.”
Using a or an depends on the sound that begins the next word.
We use a when the noun we referring to begins with a consonant (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y or z), for example, a keyboard; a mouse; a laptop; a flower; a pencil.
We use an when the noun we referring to begins with a vowel (a, e, i, o, u), for example, an orange; an egg; an engineer; an one-way street.
We use a also when the noun we referring to begins with a consonant sound: a university; a united team; a uniform.
In the same case, we also use an when the noun we referring to begins with a vowel sound. For example, an hour, an honest man.