Conditionals must be, by their very nature, unsound. They're unsound because they are non-deductive. They express what could be the case, not what is the case. What could be the case isn't what is the case.
Sometimes conditionals are about what will be the case or what is necessarily the case. These conditionals can rely on a certain amount of sound and deductive reasoning. Though even what will be the case isn't the case now. And many sound and deductive systems wouldn't concern themselves with what will be the case in the future. That wouldn't be the domain of deductive logic; even if the future could, as it were, be deductively inferred.
Many philosophers say that “future contingents” can't be either true or false. Thus this would create a problem for deductive logics.
What about conditionals which tell us what will necessarily be the case now or what is necessarily the case now? Even these cases would be about the future, strictly speaking.
That is, if I say,
“If I drop this stone it will necessarily fall to the ground.”
the falling stone (hitting the ground) is still in the future when the statement is actually made. Thus it's still a future contingent. The future it concerns itself with is just the very near future.
Conditionals don’t deal with what is the case now and what has already been the case. These kinds of necessities aren't the concern of conditionals. You can't, for example, say:
“It's necessarily the case now [i.e., 2006] that if in 1912 someone had dropped a stone it would have necessarily fallen to the ground.”
We can say that it was necessarily the case; though that wouldn’t be a conditional at all (not even on the surface).
Similarly if someone says the following:
“If someone had added 2 + 2 in 1912, then he would necessarily have got 4.”
That appears to be some kind of conditional in that it uses the argument-form “if…then...”. However, it's only an appearance. How could a particular past mathematical addition be verified? Perhaps certain adders were poor adders. Thus not every act of addition would have come out correctly. Of course we could add the qualification:
“If someone had added 2 + 2 correctly…”
Though that isn't an if-then scenario at all: it's a statement that 2 + 2 necessarily equals 4. The truth of the arithmetical addition doesn't depend on any conditions. And conditionals are, of course, about (possible/hypothetical) conditions.
Take this present-moment statement:
“When someone adds 2 + 2 correctly now, he will necessarily get 4.”
This too is about the near future; though still the future. In addition, not all additions can be verified. However, if we're again talking about the necessary truth that 2 + 2 = 4, then this has nothing to do with conditions, as such.