In English, there are two little words whose part of speech is called “article.” These are the words “a” and “the.” (“An” is an alternate form of “a” that occurs before a word that begins with a vowel sound. It is not a different word from “a.”) Although there are only two articles in English, they are very different in the way they function and what they signify. One of the articles is called the “indefinite article” (“a”) and the other is called the “definite article” (“the”). The function of these two different articles will be discussed separately below.
Indefinite article
The indefinite article, “a,” is used before a noun (or a phrase functioning as a noun) to indicate that one is talking about one thing or person out of a particular category. When we say “a cat,” the word “cat” is the category of thing we are talking about, and the word “a” says that we are referring to just one of that category, one of all the cats in the world. The word “cat,” by itself, refers to a category of thing. Putting the word “a” in front of it just narrows the category to one individual, but not to any particular individual. In this way, the word “a” can have the same meaning as “any.” If someone asks you to “pick a card,” it is the same as asking you to pick any card. It doesn’t matter which card you pick. “A card” doesn’t specify any particular card, so you could pick the one in the middle, for example, or the one on the end, or that other one over there….
It’s tempting to say that the indefinite article is used only when there is no particular item in mind when speaking, but that is not always the case. In fact, we often use it when we do have a particular item in mind. For example, if you say to someone “We adopted a cat yesterday,” you are certainly thinking of the particular cat that you adopted–the gray one with the green eyes and injured ear, for example. The one that is hiding under your bed. Here, the item is particular, but it is particular only to you, the speaker. For your listener, the information that you adopted an item in the category of “cat” is new and therefore the cat is just one within the category of cats and not specific. Choice of the indefinite article, in these cases, reflects the awareness of the speaker that the exact item being referred to is not known or not clearly identifiable by the listener.
Use of the indefinite article places something in a conversation or text so that it can be talked about afterwards with both speaker and listener knowing the exact identity of it. However, we use “a” even when we first identify something to ourselves: “What’s that on the floor? Let me see…. Oh, it’s just a piece of dust.”
Many languages do not have an equivalent for the word “a,” but, in English, using the word “a” before a noun changes one’s meaning from simply mentioning a category of things (“cat”) to mentioning an actual member of the category. In a certain sense, the article brings that thing into existence. English really has little use for singular, countable nouns with no article in front of them. If someone says “I have cat,” the actual meaning is not really something English speakers can process: “You have the category cat? What do you mean?” Of course, we make allowances for non-native speakers’ usage, but when we process the sentence, we convert “cat” to “a cat”: “Oh, you have a cat!”
Definite article
The definite article, “the,” is used before a noun (or a phrase that functions as a noun) when both speaker and listener know exactly which thing or person in the world the speaker is talking about. For example, a speaker usually feels free to talk about “the sun,” because he or she has confidence that the listener knows which sun of all the suns in the universe the speaker is talking about, i.e., our sun, the one the earth revolves around. If there is only one hospital in town, the speaker can feel free to call it “the hospital” because he or she can trust that the listener knows which hospital they are referring to. But let’s say you, the speaker, tell your friend that you adopted the cat yesterday. If your friend knows you were planning to adopt a particular cat, then using “the” would be appropriate. The friend might say, “Oh, great. Have you decided a name for her yet?” But let’s say the friend didn’t know of your plan to adopt a cat, the friend would probably respond with “What cat?? You never told me you were going to adopt a cat!”
“The” is called the “definite article,” but why is it called “definite”? It’s because it refers to things that are fully defined; i.e, things that are unquestioned as to their identity. As mentioned above, if both people know the same thing, they can use “the” to talk about it: A husband of a couple might ask his wife, “Did you go to the bank yesterday?” She will not be confused about which bank it is. But something can become definite in other ways as well. In speech, things can become definite very quickly. Once something is entered into a conversation, it can become definite to the listening person even if the listener has no real experience or knowledge of that thing or person. For example, let’s say you tell your friend that “a strange man got on the bus yesterday.” At this point the man is indefinite because he is just being introduced into the conversation. His existence is just now being established. But in the next sentence the man becomes definite when you say “The man was wearing a very tall hat.” Why is he suddenly “the man”? The man is definite now because he was introduced to the listener with the first sentence, and now he is “the man” to the listener because he is “the man that my friend says got on the bus.” Again, the listener doesn’t have to know much at all about this man and doesn’t have to actually know who he is. It’s enough that the listener knows that the speaker has introduced him into the conversation, so now he has a spot there. He is identifiable. He is definite.
It must be admitted that sometimes speakers go through an intermediate stage between using “a” and moving on to “the.” Sometimes the speaker feels the need to point at the thing that he or she introduced to make sure the listener knows what is being talked about. The speaker might say “this man,” for example, before switching to “the man.” (This strategy is used especially in writing.) One thing is clear: “the” is used when the speaker feels no need to point at or further identify the thing or person that he or she is talking about.
The explanation of the articles given here is necessarily simplified. There are exceptions and special uses for “a” and “the,” that are not addressed here. But this explanation hopes to make clear the essence of the difference between these two very important items.