误They make a rule to do morning exercises every day, so they are much stronger than ever.
正They make it a rule to do morning exercises every day, so they are much stronger than ever.
析make sth a rule是“形成规律”“养成习惯”的意思, a rule是补语,在这种结构中,常用it作形式宾语,而用动词不定式或从句作真正的宾语放在rule之后。
我们通常六点起床。误For a rule, we get up about six o'clock.
正As a rule, we get up about six o'clock.
析as a rule意为“通常”“一般说来”,常常用于概括说明某件事情, as不可换为for。