is a combination of two 副助詞（ふくじょし） (adverbial particles), だけ ‘only’, and しか ‘nothing but’. This structure primarily follows nouns, before part (B) of the sentence will have a corresponding ない based word.
He can only eat vegetables.
Only Saturdays aren't busy, so if possible Saturday is preferable.
In this group, Takeshi is the only one that isn't good. Everyone else is considerably skilled.
Why is this cup the only one that isn't 500yen?
Despite all of these possibilities, it should be noted that だけしか～ない is an expression that appears with negative verbs, far more than any of the other combinations.
may appear as though it is saying ‘only’ twice, but each particle has its own specific nuance. These nuances are as follows:
だけ - Removes one thing (from within a group), and presents it as the ‘only’ thing that fits a certain criteria.
しか - Highlights something that exists by itself as not having any alternatives.
Due to this, each meaning compounds together, with だけ saying ‘just this’, while しか adds ‘and only this!’. Reversing the order and putting しか before だけ would not work, as the sequence of selection needs to be logical.
I can take only breaks on Sunday.
I can only take breaks on Sunday.