Why should you avoid using two consecutive articles?

English has three articles: the, a, and an. The refers to a specific, definite noun. A refers to one non-specific noun. An is used just like a but comes before a noun that begins with a vowel sound.

Because articles are used to denote whether or not a noun is definite, we only use one article before a noun.

Correct: I have to do the assignment tomorrow.
Correct: I have to do an assignment tomorrow.
Incorrect: I have to do an the assignment tomorrow.

Articles are a type of determiner. Determiners come before nouns to explain definiteness, possession, or amount. We use articles to denote the definiteness of a noun.

Examples

Give it to the dog. (We are talking about a specific dog.)
Give it to a dog. (It doesn’t matter which dog.)
I read the book last night. (Use this when it’s clear which book was being discussed.)
I read a book last night. (The book that I read is not important.)
Hand me the artichoke. (There is only one artichoke that I’m talking about.)
Hand me an artichoke. (I don’t care which artichoke you hand me.)

Because the has a different meaning from a/an, you must use only one or the other. Different articles will make sense in different situations. The can also be used to talk about more than one noun. A/an is used to refer to a singular noun only. If you are talking about more than one non-specific, indefinite noun, you can use other types of determiners.

Never use more than one article before a noun because they serve different purposes.