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“I’m going to miss you so much when you go on vacation to Italy”
Excerpt FromUnder CurrentsNora RobertsThis material may be protected by copyright.
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We do not generally use the future tense "will ~" after this kind of "when", even if we are talking about the future (there are certain exceptions, such as questions). Your suggestion "... when you will go ..." might seem to be logically correct, but in fact we don't say this. There is no obvious logical explanation: it is just something you have to learn.