When you talk using desu and masu forms you sound polite.
A person who starts conversations with negative form sentences is likely to have some difficulties in communicating with others.
Why did you just say that in past tense?
The kanji and the word share one of the same meanings: form. It doesn't just mean form though, it means tense too. Like verb tense.
This has the same reading as the kanji you learned, meaning you know the reading!