When do you need "A" and when do you need "THE?"

Here's another thing we aren't really taught in school. Oh, sure, we learn about indefinite and definite articles (a/an, the), but what's behind that? WHY do we have them? That's syntax.

"A/an" introduce things we haven't seen before. That is to say, they attach to nouns (or noun phrases, if you're picky and all properly syntactical) the reader hasn't "met" yet. 

Consider this:

James approached the large brick building with a door set squarely in the center of the street-facing wall. 

We can assume that this building's been talked about before now, so "the" is logical in that position. But we probably have NOT heard about that door before, so "a" makes syntactical sense there. Likewise, "the center" and "the ... wall" are cool, because walls have centers, and buildings face streets. Nothing weird there. The only "unknown," syntactically speaking, is that door.

But now, we know about the door. So . . .

He opened the door and looked inside the room, spotting a couch along the far wall.

Now it's "the door," because the door's been introduced already. Likewise, we assume there's something beyond it -- a room, a hallway --- so it's not strange to see "the room" there. And rooms have walls, usually, so "the far wall" works, too.

But that couch. We can't really make ANY assumptions about what's inside the room (or the hallway, if that's what we see), so it has to be "a couch," because we don't "know" about it yet.  Let's keep going.

On the rightmost cushion, he spied cigarette burns and blood stains. 

Why can we say "the" there, instead of "a?" Because it's expected that the couch will have cushions (or some other kind of surface for sitting). We don't need to say "On a rightmost cushion," because we assume there will be one. HOWEVER, we'd say:

To the right of the couch stood a small glass table.

It's "a" table because, again, we haven't "met" it yet nor do we necessarily expect there to be one.

I think that's enough to provide context and make the point. This could go on and on and on.
Shared publiclyView activity