Why can't you say "Мама і донька"?
I think you should report
Technically you can, it's basically the same meaning. But as in English, мати (mother) is more formal compared to мама (mom). It's just a matter of being more or less polite :)
мама і мати both mean mother. So, both 2 and 3 should be correct, right? The question did not tell us whether there was a formal meaning or not. I did report it.
mama i donka
What's the function of ь?
I think it's supposed to "soften" a consonant sound. I think that's what I read, although I don't understand how it actually works when I am listening to the words.
You are right, "Ь" is used to soften consonants. The sound of a consonant changes pretty the same way as it does when it precedes letters e, i and y in French or German
It changes a bit the pronounciation of some consonants, for example, if you put a ь after н, it will sound like spanish ñ.
When can I use "a" instead of "i" to mean 'and'? "a" was marked wrong here.
You can find here some patterns about when to use "a": https://www.duolingo.com/comment/9012790
to me ь sounds like adding a soft "i" to the letter before the letter it is supposed to soften ie донька sounds like doinka (at least thats what i get from the audio)
That's an effect of the ь but that's not what it does. The best way to explain it is like a й(j) sound after the consonant.
Is ь silent?
Yes, the ь letter is silent and makes the preceding consonant "soft" (palatalized). We call it "м'який знак" - literally "soft sign". So you would pronounce "донка" as "donca" and "донька" as "doñca" in Spanish.
How can I use this special keyboard on my cellphone?