Wiktionary- Yesterday: "From Middle English yesterday, yisterday, ȝesterdai, ȝisterdai, from Old English ġiestrandæġ, ġister dæġ, ġestor dæġ, ġeostran dæġ"
i går: From Old Norse í gær, í gjár (“yesterday”) (whence Icelandic í gær and Faroese í gjár).
'Gårdag' also means 'yesterday' in Swedish, and it's possible the etymology is related to the Old English.
Oops! Apologies to the mods. My response was marked as incorrect and I indicated that it should have been accepted. I had "spoke to" rather than "spoke with". When I checked my answer again I realised that I had a typo ('here' instead of 'her'). Sorry about that. Time for me to get new glasses.
igår is an accepted answer. Both versions are perfectly fine, but the Language council recommends writing words like i går, i dag apart, and that's why write them that way in the course.
However there's a problem with getting the machine to accept any kind of spelling variation in the listen-and-type exercises. Unfortunately this isn't anything we as course creators can fix.