Sometimes, we use 'the' to refer to non-specific things. There are (of course) many exceptional uses of 'the', but I'm interested particularly in why we say...
The washroom/toilet (even in a building where there are obviously several, and we don't care which one). 'Where's the washroom?'
The bank, store/shops, beach and library (even in a situation where it's not clear which particular one). 'I'll give you the money later - I need to go to the bank'
In the examples, either the speaker or the listener doesn't know which particular bank or washroom is being referred to, so, according to 'the rules', the indefinite article ('a') should be used.
I've heard it suggested (by a university lecturer) that there's a regular bank or library that ones goes to, and therefore 'the'. That doesn't cover the 'washroom' example, or a one-off vacation to the beach, though. Also, even if you've always (regularly) had a brother you'd still say to a stranger, 'I have a brother'. The point with 'the' is that everyone in the conversation has to know which particular thing is being talked about.
My own idea is that it's maybe a linguistic habit. When 'the rules' were being formed, there was only one bank, beach, library etc. in each town - and every building had one washroom. Therefore we stick with 'the', the same way Londoners might still say 'the airport', although they have a choice of three. That's not so convincing either, though. In other cases, the language has adapted to, for example, multiple train stations in a city.
Is it because everyone knows quite precisely what a generic bank, beach, washroom etc. is like? 'The' in that case would signify 'you know the kind of thing I'm talking about' rather than 'you know which one I'm talking about'.
"Where's a washroom? " has not the meaning of "Where's the washroom?". The former is just a question about plumbing as "Is there a washroom here ?" would be. The latter carries the information that the speaker wants to use a washroom, true enough, but wants 'the' washroom 'the one' or 'one of the ones' that is provided or available for him or her to use.
" I must go to the bank" carries the information that the speaker has a bank or branch of a bank in mind. It is probably one that he regularly uses. He knows where it is. "I must go to a bank" is commonly heard in Britain too. It translates into "I must find a bank" as often as not.
In British ' to the beach' means just that; the beach in the place where the speaker is . We do say ' go to the seaside' . You can't have 'a' seaside anymore than you can have 'an air' in 'he took to the air'. We do say 'go to the coast' though, and you can have 'a coast'. This may prompt the enquiry " Where?" or " Which coast ?" but is usually taken as being whatever coast the speaker has in mind and 'the' indicates they do have one in mind. If they hadn't they'd say 'a coast'
Similar thinking applies to 'station' or ' tube station'. London has several mainline stations, and always has had, and dozens of tube ones. When we say " I'm off to the station" we have a defined station in mind. " Off to the tube station" gives the same indication. Someone who says " I'm off to a tube station" is not concerned which one; any one will do it seems. We may conclude that all they want is something found at tube stations in general such as a tube map or a copy of the Evening Standard newspaper.
When would you think of saying " I have the brother" ? You would if you had previously been talking of your brother or that brother and he was now with you. 'The' then defines him to others as the person of whom you spoke, the person you had in mind.
It's the mind-reading aspect of 'the' and 'a' that throws my students (most are from Asia, and Asian languages tend not to have exact equivalents of 'the' and 'a').
When I say, 'Did you feed the cat?', I use 'the' because I know that my listener knows which cat. When I say, 'I have a cat,' it's because I know that they don't know which cat, although I do.
(It also worries my students that I can say, 'I have a cat. The cat is black.' because the second time I mention it, the listener knows which cat it is[ie. 'the cat that I just mentioned']. In the blink of an eye, it becomes a particular cat.)
So... it seems that, when I say to people 'I godda go to the bank', I know that they don't know which bank, but I also know that they know that I know that they don't know which bank, but that I trust them to guess it's a particular one appropriate to the situation.
. We do say ' go to the seaside' . You can't have 'a' seaside anymore than you can have 'an air' in 'he took to the air'. We do say 'go to the coast' though, and you can have 'a coast'. This may prompt the enquiry " Where?" or " Which coast ?" but is usually taken as being whatever coast the speaker has in mind and 'the' indicates they do have one in mind. If they hadn't they'd say 'a coast'
