The teacher said he would fail me if I don't work harder.
这里would是不是用错了，如果按照if从句是一般现在时，主句应该用will。
【答】正确的句子应该是：The teacher said he would fail me if I didn't work harder.
请看句子的演变过程：
直接引语 The teacher said :“I will fail you if you don't work harder.”
间接引语 The teacher said he would fail me if I didn't work harder.
