"A" and "an" are articles.
"A" is used for words that begin with a consonant (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x and y ).
Examples: a squirrel, a key, a trunpet, a pineapple
"An" is used for words that begin with a vowel (a, e, i, o and u ) or silent "h".
Examples: an apple, an egg, an ice-cream, an ostrich, an umbrella, an hour
"The" is used to
a. mention about a person, an animals, a things or a place that is already mentioned.
Example: I have a cat. The cat is black.
b. refer to a particular person, animals, things or place.
Example: The man over there is my uncle.
c. show something that is the only one of their type.
Example: Some astronauts had landed on the moon.