Teach Us, Teacher
Past form of adjectives (Lesson 19)
You've learned there are two kinds of adjectives in Japanese, I-adjectives and NA-adjectives. I-adjectives end with the syllable I, such as YASUI "inexpensive." NA-adjectives take NA after them, when they modify nouns, such as in SUKI, "to like." When it modifies a noun, it becomes SUKINA. So, somebody's “favourite manga” is SUKINA MANGA.
To turn I-adjectives into the past negative, you change I to KU NAKATTA. YASUI (inexpensive) for example, becomes YASUKU NAKATTA (was not inexpensive). II (good) becomes YOKU NAKATTA.

To turn NA-adjectives into the past negative, you add DEWA NAKATTA. SUKI (to like) becomes SUKI DEWA NAKATTA (didn’t like).