Is "this important moment" also correct? I wasn't sure whether to report it correct or not? If not, can someone explain grammatically why it isn't.
Your version would be 'Ta ważna chwila'. This is a pronoun so it has to match noun's gender.
To (jest) is a construct, that uses abstract 'this' and that's it. That is the way we speak.
To (jest) ważna chwila. = This is an important moment.
You can drop jest, because it sounds more natural, and you get the default translation.
Cześć! Marek, I want to thank you for the IPA for "chwila". Yes, it does help. No wonder I hear 'h' and 'f' at the beginning. Dzięki!
Oh, right, I forgot about orthography. Basically, "h" and "ch" is the same sound, it's only a matter of orthography. And yeah, devoicing follows, but this is something that we're not really aware of, pronouncing 'f' and not 'v' is not a conscious decision.