"We do not come today."
Translation:Vi kommer inte i dag.
Not really. I dag is from the beginning two separate words. But, since it's such a common phrase, it has more or less transformed into one single word. The pronunciation is the same, the meaning is the same, the only difference is that people have started thinking about it as one word alone.
I wouldn't say any of the forms are informal, however, it is probably easier to remember it as i dag, since many other expressions take the same form:
- i förrgår (the day before yesterday)
- i övermorgon (the day after tomorrow)
EDIT: Turns out many consider idag less formal. I'm not for formalities myself but if you write i dag, you can't go wrong.
You are mistaken. Svenska Dagbladet generally writes it apart, see image:
The one hit I got on idag today leads to an article on whether to write words together or apart: http://www.svd.se/kultur/sprakspalt/snariga-regler-for-sarskrivning_119426.svd
Both ways are perfectly acceptable, but Språkrådet recommends writing it apart for the reason Anders91 mentions.
As far as I know, there is no regional variation whatsoever concerning which way people prefer to write it.
I certainly have. So this is different from German in which an object-verb-subject-adverb construction like "das akzeptieren wir nicht" would not be awkward, though the preservation of the word order in English would be less than desirable, though not erroneous (just Yoda-esque). Tusen tack !