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All English grammar coursebooks I have seen state that the Zero Conditional refers to something that is always true (and therefore is always certain) and has the form “Present Simple + Present Simple”.

• But if I want to say something like that but putting a focus on the fact that I have in mind some past or future situation or event, can I use a construction “Past Simple + Past Simple” and “Future Simple + Future Simple” correspondingly?

For example, I’m talking to somebody about our common friend who doesn’t believe that the lower the atmospheric pressure the lower the temperature when water starts to boil. We know that our friend was in the mountains last week and is going to visit the mountains once again in a week's time. And we know that he is kind of obsessed with checking the above-said phenomena and about a month ago announced that he would try to find any opportunities to verify whether that’s true. My question is: in this situation is it grammatically correct to say:

1. If he heated water when he was in the mountains last time it (certainly) started to boil at a lower temperature, than at 100 degrees Centigrade. He saw it for certain, because this is law of nature!


2. If he will heat water when he is in the mountains next time it (certainly) will start to boil at a lower temperature, than at 100 degrees Centigrade. He will see it for certain, because this is law of nature!

Your past zero conditional is perfectly correct. It's true that most grammar books focus on present zero conditionals, not past. However, there are some books that explain the use of past ones. For example, Macmillan Inside Out upper-intermediate does focus on them:

Sentence C is a real past conditional:

If I didn't behave well, I didn't get any pocket money. (the speaker is talking about his childhood)

So, you sure can say

If he heated water when he was in the mountains, it started to boil at a lower temperature.

However, it's not the same for if + Future Simple, Future Simple. To refer to the future, you need to use the First conditional (real future):

If he heats water when he is in the mountains next time, it will start to boil at a lower temperature.

Will is possible after if though. Here are some examples:

Consider, if you will, the position of women in Afghanistan. ("if you will" is used for asking people who are listening to you to think about something)

It’s relaxation; another form of meditation, if you will. (here "if you will" is used when describing something in an unusual way or in a way that you think someone might not agree with)

If you won’t tell him the truth, I will. ("won't" is used for saying that you are not willing to do something; in other words, "if you don't want to tell him the truth, I will.)