"This is my mother."
Translation:To jest moja matka.
If 'mama' wasn't accepted, then I guess the course creators want to show that "matka" is more formal than "mama", just like "mother" is to "mom". I think "mother" in English is a lot more common than "matka" in Polish".
You would only say this if you despise your mother. "To (jest) moja matka." uses construction to (jest)/to (są) + nominative and sounds quite normal. "To jest moją matką." uses "to" as neuter demonstrative pronoun with implied omitted noun and suggests that you don't consider your mother a human - you can't omit "jest" in this sentence. "To jest moją mamą" is very strange, on one hand you are expressing hatred by using neuter pronoun, on the other love by using diminutive. "Ta jest moją matką" uses feminine demonstrative pronoun, and as unnatural and pompous as it sounds, could be used in poetry or theatre play. "Ta jest moja matka." sounds biblical, in modern language is not used at all. Don't mix them!
This is very informative, and im now petrified of ever speaking Polish again knowing that i am probably accidentally expressing hatred for people just by being crap at using the right word endings. I had never even thought about it in this way.
I think that comment was a bit too harsh :D But generally you use Instrumental for "X is Y" (where both X and Y are noun phrases), not for "This is Y". Therefore saying "To jest moją matką" would be treating the word "To" ("It") like a noun, which makes rather little sense.