Attention: i dag/idag BUG
There's a new bug which causes the machine to say that idag is a typo for i dag in translation exercises. There was already a bug which caused this to happen in the listen and type exercises. Unfortunately this isn't something we as course contributors can fix. I've reported it to Duo, but I have no idea how long it will take for them to fix it. Both spellings are perfectly correct. The same goes for igår and i går etc etc.
Since these words are very common, this question will pop up everywhere in the sentence forums as long as the issue isn't fixed. If you see questions about it, you can link people to this topic.
The longer story about i går vs igår etc. is this:
The language council and similar organizations recommend writing them apart, although they also stress that both versions are correct.
The reason for this recommendation is that while igår, idag, ikväll, imorgon all work written together, there are some similar expressions that don't.
i övermorgon, i eftermiddag and similar can only be written apart.
Therefore it's easier and will create less confusion to write them all apart, which is what we have done in the main sentences in this course.
Apart from this bug, there is also a(n older) bug in the listen and type exercises. In these, only the exact spelling in the main sentence is accepted. So ikväll, idag etc are unfortunately not accepted in those. It's the same for a word like tv which can be spelled either tv or teve, both are perfectly fine, but only the one in the main sentence will be accepted. The only thing we can do is to disable the dictation exercises, which we have tried to do for all TV sentences, but we can't disable dictation exercises for all sentences with i dag, i morgon etc, there are just too many of them. So try to remember to write it with a space in those.
Well, both are accepted so Duolingo should probably change that, but what I have learned is that "i går" is to prefer. This is what you will see in newspapers for example.
Here are some references: http://www3.sprakochfolkminnen.se/sprakladan/ShowSearch.aspx?id=id=38363;objekttyp=lan http://www.ordkollen.se/ihop-eller-i-sar/igar-eller-i-gar/ http://info.tt.se/tt-spraket/navigation/sammanskrivning/