Think for a moment about what the words actually say, rather than what they’re most likely intended to say…
As written, the beginning of the first sentence describes the alien spacecraft, not “you” as viewpoint character. However, if the aliens are visiting your grandparents, shouldn’t they have already been there instead of just now landing in the back garden? 🙂
This example can be fixed by adding two words:
While you are visiting your grandparents, a real alien spacecraft lands in the back garden. Your grandpa seems oddly calm about the whole thing.
While visiting your grandparents, you see a real alien spacecraft land in the back garden. Your grandpa seems oddly calm about the whole thing.