Because kaf is almost never used at the end of a word, and rarely to make the k sound in modern hebrew. Kuf is much more common. Also, because the root for this word is מ.ת.ק. Using a kaf would change the meaning as well as the pronunciation.
would be nice and helpful to hear the words when selecting them
Why is it not correct to say התפוז הוא לא מתוק
Hebrew has no "to be". You can say התפוז הוא לא מתוק but it's weird and unnecessary and you shouldn't.
Okay, but I've seen a few instances on Duolingo of הוא or זה before an adjective, so how do I know when to use it?
ha-tapúz lo matók.
Hatapúach lo matoq?
The fill in the blank sentences are backward: "לא התפוז ____". Not helpful for learning hebrew.
why is the final qaf not used in sweet?
because this word does not end with a qaf but with a quf and a quf does not have a final form. The qaf (khaf) is this one: כ (and final form being ך). The quf is this one: ק
Any general rule about using tav vs yet?
Why isn't it 'התפוז זה לא מתוק' , bc its talking about 'the' specific orange?
That would be "this orange", in formal writing, which is different from "the orange".
Why did it mark התפּוז לא מתוק wrong?
Did you write it with nikud? That was the mistake. The system doesn't recognize nikud properly and rejects correct sentences. That is why we are told in the introduction not to use nikud.
הכתום לא מתוק
Should be accepted.. all the words are correct and I've long since stopped searching for sence in these sentences..
No, it shouldn't be accepted. The word כתום is orange, as in color. This sentence asks for תפוז orange, as in fruit. Yes, some sentences are weird, but what you wrote is a nonsensical sentence.