Ok, according to the grammar, we use "The" for definite noun.

But my question is:

When is a noun definite?

See this conversation:

A: I saw a big rat in my back yard last night.

B: How big is THE rat?

Why does "B" use "THE" when he has never seen that rat?

So, we can use "THE" even though we have never seen or known it before, can't we?

See other conversation:

A: Let go to the zoo

B: I want to see THE elephants

Does this above conversation make sense? So, we use "THE" because we identified THE elephants in the zoo, don't we?

But see other conversation:

A: I want to see elephants

B: Let go to the zoo

The plural "elephants" without any article in the above conversation mentioned elephants in general, didn't it?

I am so confused! When is a noun definite?

  • 5
    Full treatment of definite (and indefinite) articles could be worth a chapter or a thesis or even a book. It's particularly difficult for learners whose first languages have no articles. The most succinct explanation of "definite" I've come across is "(the speaker thinks) you know which I mean". (Keep in mind that there are an overwhelming number of curious/unexpected cases in real usage.) For more details, read the "two basic rules" in my old answer to this question: ell.stackexchange.com/questions/17430/… – Damkerng T. Sep 28 '16 at 11:18

The definite article is used to indicate a noun that is known or has been previously specified in the context. In your first two examples the first speaker has defined what noun they're talking about, so it's natural for the second speaker to use "the" to refer to that thing:

"How big is the rat (that you saw in your backyard)?"

"I want to see the elephants (that are at the zoo)."

In the third example, it's common to say "the zoo" because usually both people know which zoo they're talking about. Not many cities have more than one zoo.

Related example:

"Do you like the fruit they serve with breakfast?"

"The bananas are ok, but I'm not fond of the mangoes."

If you were talking in general about your like/dislike of fruit, then you would omit the "the":

"Bananas are ok, but I'm not fond of mangoes."

Also, you can also use "the" to imply that your audience should know which one you mean. For example:

"Could you hand me the towel?"

"Which towel?"

"That one, right there in front of you!"

Again, if you just wanted any towel, then you would say instead, "Could you please hand me a towel?" By using "the" you imply that it should be obvious which towel you mean.

I'm sure this is not a comprehensive list and that there are many other nuances and exceptions, but hopefully this will give you a good general idea.

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Let's say there's a ladybug crawling on the floor, and you want to trap it and set it free outdoors. You call to your friend:

Hand me a glass! I want to trap this ladybug and take it outside.

You used "a" to let your friend know that you do not have a particular glass in mind. You don't care which glass your friend chooses. Any glass would suffice. A tall tumbler. A short juice glass. Even a jelly jar.

When you tell your friend that you "saw a rat", you are letting your friend know that you saw a member of the species Rat. It was not a weasel or a cat but a rat. If you used the word "the", your friend (a native English speaker) would be puzzled and ask "What rat are you talking about??? Have I been daydreaming? Were you just talking about a rat?"

If you see a rat again the next night, you could say:

I saw a rat again last night. It could be the same rat I saw the night before. It's hard to tell, since it was getting dark. It was about the same size and it was wearing a miniature Boston Patriots jersey, just like the one last night.

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