U ask your mom
"This is bad." = "taihen desu."
warukitsi
Mazui.
warui ninja Warui means bad or mean
totemo warui desu
Warui o okami
you can say "KHOOB KHARAP" which means VERY BAD in Bengali.
TOTEMO furui
bad dreams warui yume nightmare akumu
Hijo ni kanashii
You may say 'Sugoku/Totemo oishii desu.'
What kind of bad? Especially if you're being polite about it (which the Japanese usually are) one might say "yokunai" (よくない) which is the negative form of good (lit. not good), or if it is really bad, like a bad habit like smoking or whatever, one would say it is "warui" (悪い) which can mean bad or evil. They are both i-adjectives.