Yesterday, when Donald Trump held his 9/11 speech, he said something like;
‘If they decide to attack us again, they will never have seen anything like what will happen to them.’
So it is clear from what he said that the future perfect can be a part of the first conditional, but why didn’t he simply say: ‘If they decide to attack us again, they will not know what will happen to them.’
I’m not a Donald Trump fan, I simply watched him on television during the news.
You are correct. Grammatically, the structure is an unusual one.
But it was a speech, after all. In spoken contexts such slips do not matter much unless a stickler like you and me takes it up. Also, American English may not care for the grammar of English as much as British English.
It’s a strange sentence. I agree. But speaking from an American’s point of view it makes sense. However, saying “If they decide to attack us again, they will not know what will happen to them.” Isn’t the same as saying what he actually said. They have different meanings. What you said means, they won’t know what’s coming. What he said means, they will have never seen such devastation or violence.
I think the sentence can import a clearer sense if it is recast this way: If they decide to attack us again, what will happen to them will be something that they will never have seen.