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"?)