dzieci is an exception.
Plural ending -a is used for singular neuter words ending in -o, -e and -um (the latter being Latin loanwords).
Is there a way to remember the difference between cookie and cake? I always get confused between the two.
In Polish, the two are related by adding diminutives. "Ciastko" (cupcake, roughly) is a diminutive form of "ciasto" (cake), and "ciasteczko" (cookie) adds yet another diminutive. Perhaps that will help.
why was 'he has some biscuits' marked wrong? How in Polish can you differentiate between 'he has biscuits' and 'he has the biscuits', which obviously have different meanings in English?
We don't. Or you can interpret 'the' as 'these' (these biscutits that we talked about earlier) and translate it to "te ciastkeczka/ciastka".
'some'... I guess it's acceptable, but technically it's "trochę ciasteczek/ciastek". Added anyway.
Sounds kinda like "one" to me... I disabled the audio exercises, but there's not much more I can do.
Yes, of course. But it does sound like that (even to me, and I say 'Sounds fine to me' to 9/10 similar comments), and that shouldn't happen, so at least no one will get the 'type what you hear' exercise.