"You do not offer her a new dress."
Translation:Tu nu îi oferi ei o rochie nouă.
I believe this is the key, yes: "ei" is linked to "îi", it specifies that the (indirect) object of the sentence (the person to whom the dress is not being offered) is feminine, not masculine (otherwise it would be "lui").
If I understood the rule correctly, the "îi" is required (in a complete sentence), the "ei/lui" not - it's only an additional information carrier or has a reinforcing/confirming function. (So "oferi ei" would not work on its own.) In the explanations in Duolingo's respective lesson, these forms are referred to as "unstressed" and "stressed (full)" forms: https://www.duolingo.com/skill/ro/Dative-Pronouns What confused me, is that the "ei" is the first word of the sentence, the position we usually identify with the subject - at least in the beginning, while we are still making very simple sentence structures. I'm not sure whether this was done here for emphasis, or if it has something to do with the negation (nu oferi).... I believe "pe ea" would signal an accusative case: https://www.duolingo.com/skill/ro/Accusative-pronouns Whether accusative (1) or dative (2) case, actually depends on the verb! And the question structure would correspond: (1) Pe cine suni? (O sun) Pe ea. (2) Cui "îi" oferi o rochie nouă? Îi ofer o rochie nouă ei, nu ţie! (I'm offering a new dress to her, not to you!) Hope this makes sense - and is correct! Am currently studying verb lists specifically to differentiate acc. and dat. cases better - happy to hear more input or corrections if anyone has some ;-)
Thank you! And would it be similar if the sentence wasn't negative? So if the sentence we want to say was "you offer her a new dress", would "îi oferi ei o rochie noua" sound more like a question, while "ei îi oferi o rochie noua" would be more natural? And in case the answer is no, would there be, this time, a difference in emphasizing different parts of the sentence? (I hope my question is clear, and if it isn't, I apologise
Both can be affirmative sentences although the first one sounds more like a question. The thing is, in everyday speech, the sentence would be just "Îi oferi o rochie noua". The ei isn't necessary". Presumably we know we are talking about a "she".
Also don't forget in speech you have intonation, in writing you have punctuation marks. So, in writing, all of the three (including the negative one) can be affirmative sentences, even though some are a bit awkward.