He is ninety if he is a day.

请教老师，这句话到底的意思是，怎么样理解？

有一种条件句叫做修辞性条件句Rhetorical conditional clauses。这种条件句使用真实条件句的形式，但实质表达的是强烈的陈述意义。有以下二类型型：

1 主句的内容显然是荒谬的，不可能是真实的，所以if从句的内容也不可能是真实的。

If theyre Irish, Im the Pope. [Since Im obviously not the Pope, theyre
certainly not Irish.]
If you believe that, youll believe anything. [You certainly cant believe
that.]
If she doesnt get first prize, shes no daughter of yours. [She certainly
will get first prize.]



2 假如if从句的内容是真实的，那样主句的内容也就是真实的。if 从句坐落于主句后。

Hes ninety if hes a day. [If youll agree that hes at least a day old,
perhaps youll take my word that hes ninety.]
The package weighed ten pounds if it weighed an ounce. [The package
certainly weighed ten pounds.]
The painting must be worth a thousand dollars if its worth a cent. [The
painting must certainly be worth a thousand dollars.]