"This sweater is not long."
For 'i-adjectives', when the adjective is used in an adverbial function or when it is negative (～くない）the い will be replaced with く.
This is not the case for 'na-adjectives'.
小さい (i) ➡ 小さくない
大きい (i) ➡ 大きくない
きれい (na) ➡ きれいじゃない
げんき（na) ➡ げんきじゃない
Not all adjectives ending with i are i-adjectives, but most are. If you know the kanji for the adjective, you can figure it out, but for the time being it is probably best just to try to remember which type of adjective each one is as you learn it.