English teacher here. Articles in English can be really confusing but essentially we use the definite article in this situation because:
Uniqueness: In most situations, there's only one mirror in a room or a home that's readily available for someone to look into.
Generality: Similar to "going to the bathroom," "look in the mirror" refers to the general act of using a mirror to see oneself, not interacting with any specific mirror.
Tell me you haven't read Jonathan Strange without telling me you haven't read Jonathan Strange 😏 obviously it's because all mirrors are connected - as entrances to the King's Way of old.
Ukrainian here. IMO, the first statement is half-stupid, the second one is half-overcomplicated :) no offense to you personally, of course. I understand the whole concept of articles in English and know (at least I thought I knew before this post) their correct usage, and in all use cases I can remember the article uses are logically acceptable for a foreigner, but this one with the mirror and the bathroom is messed up a bit :)
:) no, it's not, because for people to understand you you don't need to grasp 75% of the concepts of the English language, and IMO, this is the measure of "pain in the ass" of a language. so still not even half as complicated as Ukrainian and not even half a pain in the ass as Ukrainian :)
The second example is quite good imo. You would never say “I’m going to a bathroom.” Even if you were in a stadium with hundreds of bathrooms, you would still say you are going to the bathroom.
Same as you might say I’m taking the train. Not usually taking a train, though I’ve heard that too sometimes. Though oddly you usually say I’m taking a plane, not the plane. Also I’m taking the freeway, not a freeway. I’m usually going to the doctor, less often than a doctor.
The second example is quite good imo. You would never say “I’m going to a bathroom.” Even if you were in a stadium with hundreds of bathrooms, you would still say you are going to the bathroom.
you are justifying this statement "because of reasons" :) this is not a logical explanation. there are a bunch of bathrooms in the world. Am I going to the one specific that was mentioned before in the context? then it's "to the bathroom". is this bathroom just a random one that I will encounter on my way? then why not "to a bathroom"?
same with trains: even if you are speaking about a train route with a specific number, you are not talking about a specific train, then it should be "I'll be taking a train". if you are going to take the specific locomotive for a ride in a museum, then, obviously, you are going to take "the train". this sounds logical for the usage of Articles in the English language, at least in my head.
same with doctors: if you are going to a specific doctor, and your opponent knows which one by the context, then it should be "to the doctor", otherwise it would be a random doctor that will be assigned to you as soon as you arrive to the clinic, it cannot be "the doctor" by the same principle I always thought exists :)
I'm not even talking about the option of using a subject without an article, like in Ukrainian. "I'm going to bathroom". if I wanted to say which one, I would have stated it, but as soon as I am going to pee in a random bathroom I find, why can't it be just "I'm going to bathroom"? :D yeah, now it sounds like a verb :D
whoa, another meaningless "the" appeared. "The English Language" phrasing is used despite there are at least four? five? English languages.
so... it's complicated :D but this does not prevent English from being The Language of the World, and I cannot name a language that could have been on its place.