The indefinite article is a or an. But how do we know when to say a and when to say an?
The rule is really very simple. It depends on the SOUND at the start of the following word. (It does not depend on the way we WRITE the following word, it depends on the way we SAY it.)
If the following word starts with a consonant SOUND, then we say a :
a dog
a game of tennis
a human error
a Norwegian
a very big stick
If the following word starts with a vowel SOUND, then we say an :
an antelope
an easy climb
an interesting suggestion
an old dog
an umbrella
Normally, we pronounce consonant letters with a consonant sound, and vowel letters with a vowel sound. But there are some exceptions. The rule about a or an is still the same. You just need to think about the SOUND, not the WRITING. Look at these examples: