a and an both mean one. Use a before most words: a book, a pen, a dog.
a + most words
a book
a pen
a dog
a pencil
Try it: That is ___. (a dog / an dog) Answer: a dog — "Dog" does not start with an a/e/i/o/u sound, so it takes a, not an. Save an for words that begin with an a/e/i/o/u sound: a dog.
an before an a/e/i/o/u sound
When the word starts with an a, e, i, o, u sound — like apple, egg, orange — use an.
an apple
an eraser
an egg
an orange
an umbrella
Try it: It is ___. (an apple / a apple) Answer: an apple — "Apple" starts with an a sound, so it needs an, not a. The n makes the two sounds easier to say together: an apple.
Try it: Think about the sound. It is ___. (an umbrella / a umbrella) Answer: an umbrella — "Umbrella" starts with an a/e/i/o/u sound, so it takes an.