-았- is a prefinal ending for the bright vowel, so if the last vowel of the verb stem is [ㅗ] or [ㅏ], you can use 았. In other cases, use -었-. The verb stem of [좋다] : 좋- 좋- + -았- + -어요 = 좋았어요 The verb stem of [이쁘다] : 이쁘- 이쁘- + -었- + -어요 = 이뻤어요 (아프다, 나쁘다, 배고프다 are exceptions, -았- is used for these in modern Korean) The verb stem of [귀엽다] : 귀엽- 귀여우- + -었- + -어요 = 귀여웠어요 (The『-ㅂ다』-shaped verbs like 춥다 or 덥다 are irregular verbs, ㅂ becomes 우 when it's used with -어/아.)

don't panic! 었 is past tense. ex) 그가 죽었다. he died. we are not using 그가 죽았다. but, in denial 았 is past ex) he was not die = 그는 죽지 않았어. this is just easy rule to disticguish 았 and 었