both of katta & datta express the past tense. datta is used with noun. ex) -he was my friend. (watashi no tomodachi datta) -it was rain yesterday. (ame datta) katta is used with adjective ex) -it was cold. (samu katta) -that cake was very good. (ano keki ha oisi katta) warui is objective, so it b warukatta. if you use warukatta, you can say like "the hotel staff were impolite to customers " in english.

sorry.im chinese.

thanks all ^_^ you helped me a lot arigatougozaimasu

It's the rule. "i" adjectives conjugate that way. Learn it! waru-i --- waru-katta yo-i --- yo-katta yasu-i --- yasu-katta