Fill in the blank with the appropriate article: a, an, or the.
There are only three articles in English: a, an, and the.
1) "a" and "an" are indefinite articles. They are used to indicate something unspecific. "an" is used when the first sound of the next word is a vowel sound.
E.g. There is a ball on the ground. I need an umbrella.
2) "The" is the definite article. We use it when we have already mentioned the thing
E.g. We are not talking about the same thing. Take from the box as you need.