It never rains but it pours
 【问】 It never rains but it pours.中的but，是什么词类?
 【答】 这里的but是but that的省略形式。but that=except that(若不是，除非)。因此but(that)是个从属连词，引出例外从句。
 当主句为否定句时，往往表达“如果不……就不会”、“若……，就一定”的意思，而且在固定格式里常常省略that。在这种情况下，but(that)相当于without+V-ing。
 1. It never rains but it pours. (=It never rains without pouring.)
 不下雨则已，一下雨就是倾盆大雨。
 2. You cannot look out but you will see it. (=You cannot look out without seeing it.)
 你只要往外看，就能看见它。
 3. Virtue is the very heart and lungs of vice;it cannot stand up but it lean on virtue.
 (Thoreau，Journal，I，P.78)
 善是恶的核心;恶离善即不能独存。
 4. He never comes but that he may scold us.
 他不来则已，一来就骂我们。
 5. He never reads a book but he digests.
 他读书必融会贯通。
 6. I never hear such a story but I laugh.
 我一听到这种故事就发笑。
 7. I never think of summer but I think of my childhood.
 每当我想起夏天，我就想起我的童年。
 8. I never hear Gigli records but I think of my childhood singing-teacher. (RWRT)
 每当我听吉格里的唱片时，我就会想起我童年时的唱歌教师。
 [参见never(never...but)]