These words always confuse me. De means "it"? Deg means 'there' or 'it'? And 'det' means they? I wish Duolingo had a chart of all the words you've learned and what they mean.
De - they
Deg - you (object)
Det - it
Someone made a PDF of all the grammar notes in the tree somewhere... I can't find it anymore though :( at any rate it probably is overdue for an update!
'de' can also mean 'the'.
"Det røde huset" = "The/That red house"
"De røde husene" = "The/Those red houses"
This is only true when 'det' or 'den' is in front of an adjective.
You can go to https://www.duome.eu/SusannaG1/progress (but everyone would replace the username in that address with their own Duolingo username) and there are three tabs, showing your progress in the skills so far, all of the words you have learned, and all of the course tips+notes. It is a third-party site not officially affiliated with Duolingo but recognized by it and allowed to provide this collection of information.