例如：下面这句话为什么用with不用for:The reason why she had not stopped at the red light was most probably that her eyes had become weak (with)old age.
分析：
with是介词；而for表示“原因”时,通常用作连词,后接从句.
例如：We can't go for it is raining.我们不能走,因为正在下雨.