本末倒置

Put the cart before the horse.

Turn topsy-turvy.

eg：Writing a book review before you have read the whole book is putting the cart before the horse.

中英俚语俗语来源：大学：“物有本末，事有终始。”

同义词：轻重倒置 Turn things upside down.

反义词：拨乱反正 Put things back to order.