He is ninety if he is a day.

請教老師，這句話到底的意思是，怎么樣理解？

有一種條件句叫做修辭性條件句Rhetorical conditional clauses。這種條件句使用真實條件句的形式，但實質表達的是強烈的陳述意義。有以下二類型型：

1 主句的內容顯然是荒謬的，不可能是真實的，所以if從句的內容也不可能是真實的。

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從句的內容是真實的，那樣主句的內容也就是真實的。if 從句坐落于主句后。

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.]