I was talking to my wife (for whom English is not a first language); we had just returned from shopping, and had bought an item that I would have felt "awkward" buying by myself.
She said something along the lines of:
"If we had not bought this item today, you will have to buy it when you come back after your trip to Europe"
I corrected her and said, she should have instead said:
"If we had not bought this item today, you would have had to buy it when you returned from your trip to Europe"
She did not understand why she had to use past tense (would have had to buy ...), when we were talking about the future.
I had no answer, and was similarly surprised when I came to analyse what I was saying. What is the grammatical rule at work here?
As an aside, it appeared that the rule took the form of:
IF (SOME EVENT E DOESN'T HAPPEN): THEN USE PAST TENSE WHEN DESCRIBING WHAT HAPPENS IN ABSENCE OF EVENT E
If you had not inoculated your child against that disease, you would have had to spend a lot of time worrying about your child catching the disease...