Guess what's inside.

Out of the hundreds of characters even beginner nihongo students won't fail to spot the word for "clean".

suki

Japanese adjectives are divided into two, the i-adjectives and the na-adjectives.

I-adjectives are conjugated this way:
高い　taka-i (basic form)
高くない　taka-kunai (negative)
高かった　taka-katta (past)
高く　taka-ku (adverbial form)

While na-adjectives are conjugated this way:
好きだ　suki da (basic form)
好き　じゃない　suki janai (negative)
好き　だった　suki datta (past)
好き　に　suki ni (adverbial)

But as we can see from the above scenes from "Cafe Relish ni Oide" the na-adjective suki is treated as an i-adjective.

Thus:
好き　suki
好くない suki-kunai (negative)
好かった suki-katta (past)
好く suki-ku (adverbial)

Notice too that the okurigana "ki" き disappears during conjugation, eventhough it is still pronounced. Not exactly grammatically correct but I guess you can't stop language from changing.