These words are a special part of speech known as theย article. It helps us differentiate which one of the things that we are talking about.
The
You useย the when you are talking about a noun that is very specific. If you say โthe schoolโ, you have a particular school in mind.
It could be the middle school that you attended when you were in 7th grade, with a specific name and something that applies to only that school. You would not be talking about any other school. This helps the listener or reader know what you are talking about, especially when you use a common noun.
A
When you sayย โa bookโ, you are not talking about any single book in particular. It could be any book that you see, from a large dictionary to a picture book.
If your teacher says, โPlease bring a book to class,โ you could bring any book that you see and that is convenient for you to have. Even if it is in another language, it would still fulfill your teacherโs request.
An
An is just a special form forย a. Because it would be difficult and confusing to say two vowels at once (a apple, for example),ย an is used to replaceย a when the next word starts with a vowel sound.
Note that this is not always the same as a word that simply has a vowel at the beginning of the word โ the vowel has to alsoย sound like a vowel. Take the wordย unicorn.
The letterย u is a vowel, but the first syllable of the word is pronounced likeย you, with aย yย consonant sound, not with anย uh, or shortย u vowel sound. For that reason, we would sayย a unicorn, not an unicorn.
Using Will or Would in a Sentence!ย