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although both meaning of "only," but it is better do not to interchange them. the most differenc that "只有" (zhǐ yǒu) is usually placed in the first half of a sentence or at the beginning of the sentence: "只有我知道,别人不知道" ("Only I know, others don't know"). on the other hand, "才" (cái)placed in the latter part of a sentence: "妹妹有两个苹果,我才有一个" ("My younger sister has two apples; I only have one"). such two words to use.