How do I determine the size of set/class, while choosing an article?

Question

Here is an example from a test on articles (1st, 2nd):

I recommend you eat ___ apple pie at this restaurant.

The correct answer is the.

I deem that it is because they are considering the set of different meals, and in this set in that particular restaurant there is only one apple pie. In other words, in the menu of the restaurant you can find only one apple pie.

But on the other hand, I could consider a lot of "instances" of the same pie ever made in this restaurant, in which case, I suppose, I should have used the indefinite article.

My questions are

1. am I right about the reasons of using the apple pie?
2. how can I choose the set correctly? For example "Taking into account the plan of your apartment, you will definitely get a fast and strong WiFi connection provided that you buy ___ Google WiFi router". Is the set of companies producing wifi routers (google, mikrotik and so on...) considered here, in which case google wifi router is a single item on the list, or the set consisting of all instances / devices produced by all companies considered here, in which case I recommend buying one of these devices?

Answer 1

Try the apple pie if you eat at Joe's Diner.

We cannot even say that there is only one recipe for apple pie followed at that restaurant. the in this context means "apple pie served there" (as distinct from apple pie served elsewhere). The recipe might vary. They might bake many apple pies every day. The person recommending the pie is saying "apple pie served at Joe's Diner" is consistently very good.

With respect to your question "how to choose the set correctly"...

You are thinking about this in the wrong way. The article reveals your thought. The article is not your master. You are master of the article.

Does Acme Gizmos make several routers or just one? If they make several, are they all equally good? Let's assume you know all there is to know about routers, but your listener is a complete newbie and knows nothing.

Let's assume Acme Gizmos makes several routers, all equally very good.

Get an Acme router.

Get any Acme router, you can't go wrong.

Now let's assume Acme makes only one model of router:

Get the Acme router.

In this one model context you could also say:

Get an Acme router.

But you would not be giving your listener the additional information that there is only one model to choose from. You're saying only that you like Acme. Your listener, when shopping, might be surprised to learn there's only one Acme model to choose from.

Now, let's make it more complicated.

Acme makes multiple routers, several quite bad, but one very good. You have the very good one in mind, the one that all the reviews have praised; everyone who knows anything at all about routers knows the others are bad. You momentarily forget that your listener is a complete newbie and does not read reviews of equipment:

Get the Acme router.

P.S. Your newbie might buy the first Acme router he sees, and it could be one of the bad ones. He might think there is only one Acme model to choose from. But you said the Acme Router because you had the good one in mind.