Re: back-shifting in indirect zero conditional sentences
I am not very happy about the traditional labels for conditional sentences. However, if one must use them, I'd use:
If she heats this piece of ice, it will melt. - First conditional, future possibility.
If you/one heats ice, it melts. - Zero Conditional, general truth.
If you/one heats ice, it will melt. - Zero conditional, general truth.
Will in the third sentence is a modal expressing certainty about a general-time situation.
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