"Sometimes I am sad."
Translation:Czasami jestem smutna.
It's because most of the adverbs in Polish are simply a version of an adjective, and Polish used to have a completely different pattern of adjective declension – that means, there are still active the so-called "short forms" of a few adjectives (which are simply the older pattern declensions) and that in turn produces a few "short version" adverbs that come from these adjectives. This is the case with „czasem” and „czasami” – they mean the same, but conform to different declension patterns.
If it helps, think of them as "You canst" vs. "You can", but the old declensions for adjectives are more popular in Polish than the old verb conjugations in English, as the change is more recent in Polish than the verbs change in English.