Ok, I get "ert äpple" but "erat äpple"? Maybe it's too late and I can't really recall where does it come from, but it would be nice to know what kind of a construction "erat" is. Sorry, if that's silly.
"erat" is regional slang which has become common throughout larger parts of Sweden. It means the same, but it's not acceptable in written standard Swedish.
Why not "er" and only "ert"? I thought er was singular for -ett words, ert was plural
No, er is for en and ert is for ett. Both mean the plural "your". So:
- er bok = your book
- ert bord = your table
I may be wrong on this as I'm just a beginner myself, but I believe 'Du' is if you were talking to a single person and 'Ni' a group of people. They mean the same thing but they're situational. So it's the same with 'din' and 'er' e.t.c. (I may be completely wrong on this and I could just go to Google to find the right answer but it's nice practice to try and work things out myself haha)
Oh, yeah that makes sense. I think you might be right. I asked my dentist (she's Swedish) and she said that din (for a singular noun) is singular and dina is plural for a singular noun, etc... (It's SUPER hard to explain). So you were right. Like the Duolingo translation is "Dina boker" (I spelled it wrong) Which is "your" (singular) books (plural). It is kind of confusing. I probably messed up on my translation but I think that you starting to make me comprehend this matter more. And if you think about it, it isn't that hard to understand. It's just hard to explain.
I picked "ditt" äpple where i also had the "ert" option and it was accepted as ok.
both are right because ditt is for you (singular) and ert is for you (plural)
So are "en" and "ett" words just something you have to memorize like "el" and "la' words in Spanish?
Both work – ert if it belongs to more than one person, ditt if it belongs to one person.