In Finnish we also say 'vartti' meaning the same as the Swedish 'kvart'. It's a lot shorter and easier way to express 'fifteen minutes' xD I think it's Swedish where it comes from.. And for anyone learning Finnish out there, this is only in the spoken language, 'puhekieli' (the written language being 'kirjakieli'). I don't know if it's slang so it might be used just in the area I'm from. Though I'm quite sure sure it will be understood in other parts of Finland too. :)
My answer was "They're coming in a quarter-hour", which was accepted and I agree with. I don't believe just "quarter" should be accepted, because the Swedish word always means a quarter of an hour and never means a quarter of other things. In English "quarter of an hour" or similar is used.
If talking about half of something, I wouldn't say 'a' before 'half'. For example, "in half an hour." If someone said, "in a half of an hour," I would understand what they meant, though. But if talking about a third or quarter, I would say the 'a'. For example, "in a quarter of an hour." If someone said, "in quarter of an hour," without the 'a', it wouldn't sound very natural to me.
Native English speaker (British) and 'they are coming in quarter of an hour' works in spoken English to me. Same as you would say 'they are coming in half an hour', you could also say 'they are coming in a half hour' but that would be a more clunky or possibly quirky way to phrase it.
I believe the equivalents of "om" would be more like "if" and "about." Nothing is going to be an exact translation. Why(in english) do we say "IN half an hour" anyways? We aren't literally going inside some sort of time dimension where we go find someone called 'half-an-hour'. It's just the word we use. Om is one of the words Swedish uses for time, that's all.
And if I'm not mistaken, "om" is used the event is occurring after that amount of time has passed, and "på" is when its during until that time is over.
Jag gör det om en vecka - I will do that in a week
Jag gör det på en vecka - I will do that for a week
Please someone correct me if wrong.
I would rather translate "Jag gör det på en vecka" as "I will do that in a week" as well, but in the sense that "It will take me a week to do it". I would translate "I will do that for a week" in the sense you gave as "Jag gör det i en vecka." Which literally translates to "in a week".
Prepositions are confusing.
The official spelling is "de" which is officially pronounced /dɔm/. Spellings don't change as fast as pronunciations. It happens constantly in language, not just Swedish.
Informally / colloquially, it often gets spelled "dom". In some dialects it is pronounced more like "de" /deː/ which matches the spelling, or in other dialects it sounds more like "dee" /diː/, but written word is more standardized and uniform across distance than spoken words are. (true for any language)
inom does mean less than or equal to a quarter, yes.
om is an approximation. Technically, it means after exactly a quarter, but nobody uses it like that - I mean, if you say in fifteen minutes and arrive after fourteen or seventeen, everyone will think that's perfectly normal. But it doesn't mean after at least fifteen minutes, no.
OK, but three other mods have claimed it's always a quarter-hour. That includes in this thread so I'm not sure what examples you are referring to. There are some examples using kvarts instead of kvart and compound words with kvart embedded but every comment I've read in the past four years led me to believe kvart is always for hours. I guess it's just almost always for hours?