Follow up question - Is "Ni" (and subsequently er/ert/era) reserved for talking to a group, or can it also be used when speaking formally one-on-one (as is the case in many other languages)? I've read that "Ni" has fallen out of usage as a formal pronoun, not sure if that was accurate or not? Thanks.
I think we are on the same page. You are merely adding that it is not all-inclusive for singular cases, correct? If so, I was aware, but failed to add that.
What I was trying to get at is this: if "t" is used, it is NOT used in a plural case. Yes?
EDITED TO ADD:
In the cases of "ett agg" and "ett brev" for examples, is there a suggestion that gender is applied?
One thing I've found that really helps me with this is to try to learn the word in the definite form, rather than in the indefinite form. For example: flicka (girl) - I learn it as 'flickan' (the girl) ... or ... hus (house) - I learn it as 'huset' (the house)
That helps me to get the gender and endings thoroughly attached to the word.
You can say Hon är ditt barn or Han är ditt barn, but that's not what the sentence means – det does not refer to the child, it's a placeholder pronoun. See this thread for a longer explanation: https://www.duolingo.com/comment/9708920
For a short fix, consider what you'd say if there was a knock on the door. You can't see the person and someone asks you who you think it is. You wouldn't say 'She's your child' but you could say 'It's your child'. (more likely of course, It's your daughter, but still not She's your daughter)
Although barn is an ett word, we actually normally don't use det as a pronoun about children. So no, it doesn't work like that.
Think of "ditt" as being used in a conversation with one parent and "ert" as being used in a conversation with both parents. In English we only have one "you", but most European languages have a singular you (for talking to one person) and a plural you (when talking to an audience of multiple people).
This has been discussed a bit already in this conversation chain - please read other comments in the future, you might find your answer there.
"Ert" might be used because the owners of the child are more than one (you maybe talk to a couple and ask them if the child is theirs). It could also be that you use the polite and slightly old fashion "ni" instead of the normal "du" when you talk to a single person. But if you are talking to a single person, you can use "ditt".
Just to expand on this great explanation - child is singular, but the possessive pronoun matches the ownership of the people whose kid (s) you're talking about. Regardless if it is kid or kids, you would use some form of "er" if you're talking directly to the parents/guardians, just like in English (for instance, if you're talking about HIS kid, you wouldn't use the pronoun "their").