Does it matter whether you use an indefinite article or a definite article when specifying a thing in your writing?
The reasons are less easy to explain, but I’ll take a shot anyway. Keep in mind, I’m talking about writing, here. Not speech. Writing.
“The” indicates something (could be a person or an item) that’s already understood or has already been discussed or mentioned.
“A” and “an” indicate something that doesn’t fulfill those requirements. Something that we cannot readily understand or define, something that hasn’t been discussed previously. (Like “a person” and “an item” in the previous paragraph.)
Let’s say we’re walking through a house we’ve never seen before. Maybe we’re thinking of buying it. Before I go further, look at that first sentence. “Through a house we’ve never seen before.” Not “the house.” “A house.” We don’t know this house yet. However, it’s “the house we’re thinking of buying” (for the sake of argument); we’re only looking at one, and this is it, so it’s “the house we’re thinking of buying.” If we were looking at more than one, this would be “a house we’re thinking of buying.” (Or perhaps “one of the houses,” if there’s a specific set of houses we’re visiting with an eye toward purchasing.)
We enter through the front door. There’s only the one, and we can see it and understand its purpose from experiences with doors, so “the front door.” Inside, we see a hallway leading to the back of the house, and another hallway crossing it in the middle. (Work with me here. I’m not an interior designer, and these constructions are purely to illustrate a point—not the layout.) Because we can’t say any more about these hallways yet—we don’t know where they lead, for example, other than the one going “to the back”—there’s a hallway and another hallway. Once we know more about them, perhaps one will become “the main hallway” and the other “the bedroom hallway” (if, let’s say, it leads to bedrooms on either side of the house).
In the big room at the front (there’s only one big room, from what we can see, so “the big room”) there’s a large window facing the street, a smaller window on the side wall facing the yard (if we said “a side wall,” folks will wonder how many other side walls there are here), and a doorway leading to what looks like a kitchen. We can’t know it’s “the” kitchen yet. And while we can see the windows, we haven’t seen them previously so we use “a” to indicate the unknown quality of them. Now that we’ve seen them, though, we can say “Jane walked over to the small window on the side wall.” If she did that immediately on entering the room, before any description has been given, we’d say “Jane walked to a small window on the side wall.” We didn’t know about that window before. Walls are more easily assumed.
“Hey, there’s a little door in the wall out here next to this cabinet.” (Hey, that room was a kitchen! Now it’s “the kitchen.” But it’s odd, seeing a little door in that position.)
“Aha! That’s for a dumbwaiter, I’ll bet.” (Whoever said this has an idea what that little door probably is, but can’t be sure until they investigate.)
If the person responding were the realtor showing the house, the answer would probably be “Yes, that’s for the dumbwaiter.” They know what it is, so they can use “the” to indicate it.
A couch sits in the middle of the big room. It’s covered in leather. The leather couch is black, but the layer of dust covering it makes it look gray. (It’s “a couch” when we first see it. Once it’s been mentioned, it becomes “the couch.” Someone probably dragged a finger over the surface and came away with dust, so “the layer of dust.”)
I could write more on this, but I suspect there’s enough here already to illustrate the point. Indefinite articles are for undefined, unknown, uncertain items/people (also for abstract nouns, like “an uncertainty” if it’s a vague feeling). The definite article is for defined (funny how that works), certain items/people (and also for specific abstract nouns, like “the uncertainty that comes with unemployment”).
Ask questions in the comments, if you like. This is a tricky subject, and if you need clarification, I’ll do my best to provide it. (“The comments,” because they’re a known item; blogs have comment sections. You may have “a comment,” so called because no one but you knows what it is until you write it.)