I know Duolingo can't fix the TTS, but it's really annoying that she pronounces this statement as an interrogative. Every time that happens, I end up translating the statement as a question. I guess I need to be more careful and always ignore the TTS and just focus on the punctuation.
So I understand that столи is a mistake, but there is an inconsistency with what Duo accept and doesn't. Previously, -и was considered a "typo", I could understand why and I would also understand if it was a mistake, but now this is a mistake.
Please, Duo, make up your mind. Is it a typo but accepted, or not accepted ? Regarding how ш and щ are not considered typos when mixed up (and much easier to mix up, to me), I would advise making и and ы not typos either but incorrect answers, but not half of the time. All the time.
It is confusing, I'll try to help: there are two forms of это.
1. Это means "this is" or "these are", depending if you are talking about one singular thing or plural things. Это never changes, it doesn't 'agree' with the rest of the words in the sentence. It just sits there, meaning "This is." ("Это -- Спарта!")
2. BUT... then there's этот, эта, это, эти. These words mean ONLY "this", indicating the noun: this tree, this book, this whatever. These forms have to 'agree' with the noun: masculine, feminine, neuter or plural. It's just kind of too bad that the neuter form это looks exactly the same as это that means "this is".
So when you see Это at the front of a sentence and it looks wrong, try translating it as "This is..."