翻译：never dreamed you'd leave in summer 有一点不明白

you would leave(would后面加原形)...
做梦都没有想过你在暑假期间就要离开..

you'd=you would ,would是情态动词，后+动词原形

you'd 是you would 的缩写 would 是能愿动词。情态动词后加动词原型

没错，就是虚拟语气

you'd=you would.