Articles are similar to adjectives because they modify nouns. The English language has two articles: the and a/an. The is used to refer to specific nouns; a/an is for non-specific nouns. Therefore, we call the the definite article and a/an the non-definite article.
In many ways, the choice of which article to use belongs to you, the writer. You must decide whether you’re referring to something specific or something vague. Using the with a specific noun means that you have a particular object (or noun) in mind. Using a with a non-specific noun means that you don’t have a particular object (noun) in mind.
A) Let’s go see the movie.
B) Let’s go see a movie.
Knowing when to use the or a/an simply relates back to the meaning you are trying to convey. In example A), a specific movie is being referenced. However, the movie in example B) could be one of many, as if you haven’t decided what movie to see. In other words, using the refers to a specific item, while using a/an refers to one of many.
Since articles modify nouns, you should also keep in mind that the different noun classifications may or may not require an article. For non-count nouns, you would use the, or sometimes an article may not even be needed. For example, “I swam in the water,” sounds better than “I swam in water,”; that is, if you’re referring to a specific body of water. However, the second sentence is grammatically correct, and works if you’re not talking about a specific body of water.
A) Please hand me bottle of water.
B) Please hand me a bottle of water.
Count nouns require an article or another sort of determiner before it to make it work. For example, sentence A) is incorrect, as bottle of water is a count noun and cannot stand alone. Therefore, sentence B) is the correct version.
Abstract nouns are tricky, as sometimes using articles in front of them are okay, and others it is not. For example, one might say, “Time is on my side,” rather than “The time is on my side.” However, in the following sentence, an article is definitely appropriate: “The time we spent together was amazing.” Here, the writer is referring to a specific period of time, rather than an indefinite span of it.
Keep in mind that mass nouns, proper nouns, and plural nouns don’t require articles, but countable singular nouns always do. Again, as the writer, it’s up to you whether you use the or a/an—just make sure that your article choice is consistent with the meaning you are trying to convey.