"Is carrot juice tasty?"
Translation:האם מיץ גזר טעים?
I don't think there is a specific reason for it, one rule of thumb that I can give you though is that usually fruit names that are derived from foreign languages used in the singular form:
מיץ מנגו, מיץ פסיפלורה, מיץ אננס, מיץ בננה
When I think about it, most of the names are used in the singular form except for some exceptions:
מיץ עגבניות, מיץ חמוציות, מיץ ענבים, מיץ אכמניות, מיץ דובדבנים, מיץ תפוחים, מיץ תפוזים
There are definitely more, these are the ones that I can think of right now.
I'm going to guess that your sentence means "tasty carrot juice?" So we need something to add the word "is". The two suggested answers are to add האם first, or to put זה in the sentence, saying ?מיץ גבר זה טעים. But I'm not sure. BTW, I answered exactly as you did, and it was marked wrong.
Because there's no direct translation in English. The only meaning האם has is to mark the sentence as a question instead of a statement. Let's say the sentence didn't have a question mark (question marks aren't spoken after all) then האם would change what would otherwise be a statement into a question. Rising intonation at the end of a sentence in spoken Hebrew also marks the difference between a question and a statement so האם can be omitted.
how to get tips and notes:
organized by skill in one pdf for the whole course: https://www.docdroid.net/JnfmyEV/tipsnotesbackup.pdf
Replace your username where USERNAME is for information on your progress & the tips & notes: https://duome.eu/USERNAME/progress
The whole course tips and notes are here (and the site has one for each Duolingo language): ,organized by skill individually: http://duolingo.wikia.com/wiki/Hebrew
You can also get them on the duolingo.com site (make sure your browser is in desktop view mode so you can see the other duolingo features like discussions & timed practice (the language clubs are only on the app though). From discussions you can search for all the discussions in this course (like this one you're reading, as well as see general duolingo discussions). If you can't see discussions on the main screen once logged into duolingo you're not in desktop view.