N しか ～ませんしか — only (with negative)
Only しか / limitしか + a negative verb means “only / nothing but”, stressing how little there is.
わたしは ひらがなしか わかりません。
I only understand hiragana.
しか always takes a negative predicate but the meaning is positive-limiting: “only”. It replaces は/が/を (ひらがなしか), and comes after other particles (ここにしか ありません).
Compare だけ (neutral “just”) with しか〜ない, which adds a nuance of “that's all, not enough / not more”.