If we remain inveterate smokers,it is only because we have s

问题补充：
if引导的条件状语从句,主句是一般将来时,从句是一般现在时.那这里为什么用 it is only because
If we remain inveterate smokers从句是一般现在时
下面如果说,we 怎么样,就会说we will .用一般将来时.
但这句没说,说了别的.只用了规则的前一半,
we have so often experienced the frustration that results from failure
这是完成式.
If we remain inveterate smokers,条件状语从句.
it is only (because we have so often experienced the frustration that results from failure.)
这是because引导的表语从句.
其中
we have so often experienced the frustration (that results from failure)
frustration后面跟了个定语从句.

no