She would have passed the exam, except that she was ill on the day.

except 是介词，意思是"除了...之外"，表示排除某个特殊情况。 that 在这里是连词，引导一个从句，说明具体排除的情况是什么。 except that 是一个固定搭配，意思是"只不过、只是"，用于引出导致结果不同的例外情况。