しか is always used with negative predicates. When a number or quantity word(e.g., すこし[a little]) is followed by しか, it emphasizes the speaker's feeling that the number/amount is small or not enough. When a noun is followed by しか, the meaning is the same as Nだけ Vます(～ only N). When しか is used with other particles, しか is placed after the other particle(e.g., Nにしか, Nとしか, Nでしか).
ゆうべは 三時間ぐらいしか ねませんでした。
I only slept for three hours last night.
ほしい 物は たくさんありますが、お金は 少ししか ありません。
There are a lot of things I want, but I have very little money.
きょうしつには ジョンさんしか いません。
There is nobody but John in the classroom.
わたしは 安い 物しか かいません。
I only buy inexpensive things.
電話は じむ室にしか ありません。
The telephone is only in the administrative office.
Lesson 6-10「学生は 日本人だけですか。」
Are the students only Japanese?/Are all the students Japanese?