Definitive and Indefinite Articles

We use "THE" before a singular noun, when we talk about an specific noun (object)

e.g.: The pencil is red and big.

We use "A or An" before a noun, when we talk about any noun (object)

e.g.: A ruler

An eraser

We use the article "A" before a singular noun beginning with a consonant sound

e.g.: A whiteboard

We use the article "An" before a singular noun beginning with a vowel sound.

e.g.: An eraser