Serious, Terrible, Very, Difficult, Hard, Hectic
adverb, な adjective
A big change is a really serious thing. And people don't like change, either, making it a terrible event. Also, in more recent history this has come to mean very as well.
たいへん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
えー、めっちゃ大変だったんだね！
Oh, it was really tough, huh?
皆様には、大変心配をおかけいたしました。
I apologize for causing you all so much concern.
日本語をマスターするのは大変ですが、がんばってくださいね。
Mastering Japanese is difficult, but do your best!