I believe the grammar in the translation is incorrect here. It should be "You or she?" If you ask, "Who is coming, you or she?" ("you or her" would be incorrect grammar in this case), then the answer in Hebrew is את או היא, but if you ask, "To whom does this belong--you or her," the Hebrew response should be לך או לה (both in the objective case).
It is and it isn't, professor. Who's to say? People say it that way, and... VOX POPULI, VOX DEI. The only context I can think of in which people use the nominative is when speaking on the phone answering the question, "May I speak to Ms. so-and-so?" - "yes, this is she.", which I have always found somewhat stilted and unnatural, an archaically sounding formal idiom, exactly just like "you or she" or the worst of them all, "Who, I?". FYI, "her" here is an emphatic pronoun, a sort of an abbreviation for "her person" or some such.
All of the Hebrew vowels אהוי can be used as both vowels or constonants. There are certain 'usuals', like ה at the end is a vowel, א in the beginning is a constonant... However, all rules have exceptions, and as irritating as it may be, there is no better way than to just practice and enrich your vocabulary
Can "את או היא?" be used whether we're asking about the subject or object in Hebrew? I mean, would it fit in after the sentence "Who saw him?" and after the sentence "Whom did he see?"?
In general, can we use "היא" for "her"? I'm looking at https://en.wiktionary.org/wiki/Appendix:Hebrew_pronouns and it looks to me like Hebrew uses a suffix instead of a pronoun to mean her.
I know that in English, we usually don't distinguish between object and subject when we say a little sentence fragment like this, so I'm not worried about the acceptability of the English.
In Hebrew, the subject form is "היא" and the object form is "אותה". While the answer to "Who saw him?" could be "her" = "היא", the answer to "Who did he see?" could not be translated from "her" to "היא" but should be translated to "אותה" instead. Here, "אותה" is the feminine form of "את", the preposition.
You can look at the translation of a preposition here: https://en.wiktionary.org/wiki/%D7%90%D7%AA#Hebrew. It has no parallel work in English.
I am a beginner with trying to spell these words off the top of my head with nothing to go by. I am not yet familiar with the Hebrew keyboard, much less the spelling of the words. This has suddenly become quite tedious. I am rethinking about how far with this I may want to go at this rate. My suggestion would be to keep the spelling of these basic words accessable until we are able to go on our own,
I felt the same way. If your course is the same as mine, you'll be able to get past this tedious phase where you dread the typing questions. When I was in that phase, I took some deep breaths, and took more breaks, and I got through it. Once in a while I skipped ahead to level 1 of the next lesson rather than level 3 of that lesson which is where the typing things were for me, but I went back to do the typing before long. Now I'm on the 12th lesson, and typing doesn't scare me. Sometimes I type without looking at the keyboard. I wouldn't be surprised if Duolingo changes this, but know that you can get through this.
If anyone is reading this who is having trouble with the alphabet, the Memrise course helps a lot.