I can't find my hat. I think I must buy one.　(不定)我找不到我的帽子了。我想我该去买一顶。The hat you bought is bigger than that I bought.　(同类但不同个)你买的那顶帽子比我买的大。I can't find my hat. I don' t know where I put it.　( 同一物)我找不到我的帽子。我不知道我把它放在哪了。