When I see the word "bastun," I think "basement." Hopefully, I'll get it straight soon.
Ljummen, maybe. The dictionary says it means lukewarm, which is a word I don't know.
If you're taking a bath (in a tub), "vattnet är ljummet" probably means that it's too cold, but if you're swimming in a lake for example, "vattnet är ljummet" means that it's warmer than average. At least in Sweden :).
Lukewarm is almost a negative way to describe something. It means it's not very warm, but it's not actually cold, which definitely fits the bath/lake description. It can also be used figuratively to mean something wasn't that well received: "Despite huge sales for his first book, the response to the author's new book was lukewarm." Can 'ljummen' work like that, too?
Can anyone tell me how to say "I'm going to take a sauna" in Swedish, please?
You can argue that the meaning is similar, but "the sauna is hot" would be ignoring the word "i" in the Swedish sentence. "It's hot in the sauna" suggests that the speaker is in the sauna, or has been recently, but "the sauna is hot" is just a passing observation.
Also, the common Swedish construction with "Det är..." maps really well to the common "It is..." in English, so I think it's almost always the best way to translate it.
Doesn't really answer your question, but Wiktionary has this to say:
"Nouns ending on unstressed (short) -u are rare in Swedish and most of them are loanwords, and the plural form -r given by normative dictionaries feels unnatural to many speakers. For this reason, alternative plural forms may be found."
I think it's an odd plural cos it's an odd word to start with.