Adverbs – Sometimes

In English we can say that something  is “very sweet” or “not very sweet”. The adverb “very” need not change. In Japanese, the adverb can change depending on whether you are saying something in the affirmative or in the negative.

ときどき (tokidoki) means ‘sometimes’. This adverb is only used in the affirmative.

ときどき おもしろい (tokidoki omoshiroi; sometimes interesting).

ときどき むずかしい (tokidoki muzukashii; sometimes difficult).

Apparently, one would not say ときどき むずかしく ありません (tokidoki muzukashiku arimasen; sometimes not difficult), because for a negative adjective, you would need to use an adverb that works for negative adjectives, and ときどき is not one of those.