Use "a" before a singular noun that starts with a consonant sound:
He wants to buy a car.
I need a pen to write this down.
She ordered a sandwich for lunch.
I saw a boy running in the park.
We need a hammer to fix the shelf.
I want to eat a hamburger.
A hungry dog will eat anything.
Use "an" before a singular noun that starts with a vowel sound:
She has an umbrella for the rainy days.
He is an actor in the new movie.
An elephant is a big animal.
I had an idea for the project.
She wore an orange dress to the party.
An / A herbivore is an animal that only eats plants.
Can you wait for me for an hour?
An honest person always tells the truth.
Remember that we choose a or an according to the sound, not the spelling. Sometimes a word begins with a vowel letter but a consonant sound. In that case, we use a instead of an.: