What is this case of using “the” in front of a noun?

As I have been taught, apart from being a part of a name, the definite article in English is usually used for:

1) things that have already been mentioned earlier in conversation (like in "So you did take care of that mother, but what about the child?");

2) things that can be seen in the moment of speaking ("Look at the driver!");

3) things that are easily understood from the context ("We went fishing yesterday. The weather was terrible!");

4) things that are unique in this world ("The Sun is shining. The air is fresh.")

However, I've noticed that some nouns that don't fall into any of the four categories above are still used with the definite article:

"We went to the forest yesterday and..." - the first phrase written by a forum member in a forum thread. There was absolutely no conversation prior to this phrase and, therefore, there is no context provided yet. So, it automatically excludes case 1 and 3. The readers can't, of course, see any forest, which effectively excludes case 2, too. And there are many forests in this world. So, it's not case 4 either. If so, then what cases is it?

Here are some other examples:

"For whomever it may concern: please, note that, unlike in Europe and
the USA, school students in Asia always have their school lunch in the
classroom."

(Why not "in a classroom"?)

"Five irritating things that can easily happen to you at the airport:"

(Why not "at an airport"?)

"This species can be found in any river or a pond. In fact, it can
also be found in the sea."

(Why not "in a sea"?)

1 Answer
1

According to the Cambridge Dictionary, you have missed another case:

the

used before some nouns that refer to place when you want to mention
that type of place, without showing exactly which example of the place
you mean

We spent all day at the beach.
Let's go to the movies this evening.
I
have to go to the bank and get some Euros

I think that the forest, the classroom, the airport and the sea in your phrases perfectly match this case.

Also, you may be right pointing out that the use of 'a' is fair in some of those phrases.

a

used to mean any or every thing or person of the type you are referring to

An airport and a sea fulfill this meaning but in my opinion you can't use a in your other phrases. They went to a specific forest. Asian students do not lunch in some classroom, they lunch in the same classroom where they are taught. In Spain at least, in schools students do not change classroom, different teachers come to the classroom.

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