The 副助詞（ふくじょし） (adverbial particle)
しか is often compared to
だけ, in that it also expresses ‘only’. However, unlike
だけ,
しか can only be used in 否定表現（ひていひょうげん） (negative statements). Due to this,
しか～ない is regularly translated as ‘there’s only (A)’, or ‘there’s nothing but (A)’.
しか～ない may be used to express numbers, or nouns. In this lesson we will examine numbers.
To use this construction, the number that you want to express as being ‘all that exists’ will go before
しか, then
しか will be followed by a verb in its ない, or ありません form.
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今日（きょう）は5キロしか走（はし）れない。
I can only run 5 km today.
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今日（きょう）は100円（えん）しか持（も）っていない。
I have nothing but 100 yen on me.
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今日（きょう）はたくさんの宿題（しゅくだい）があるから2時間（じかん）しか遊（あそ）べない。
I have a lot of homework today, so I can only hang out for 2 hours.
The main difference between
しか and
だけ, is that
だけ suggests that (A) was ‘chosen’ from among a group where several other choices exist.
しか on the other hand implies that there are no other choices, and (A) is the only possibility/thing that exists.
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冷蔵庫（れいぞうこ）の中（なか）には色々（いろいろ）な物（もの）が入（はい）っていたけど、リンゴだけを取（と）り出（だ）した。
There were a lot of things in the refrigerator, but I just took out an apple.
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冷蔵庫（れいぞうこ）の中（なか）にはリンゴしかなかったから、リンゴを食（た）べた。
Apples were all that was in the refrigerator, so I ate an apple.