|This doesn't sound like a Nested IF, it sounds like a basic IF function.|
You also didn't tell us what to return if the "certain cell" isn't empty
Let's assume your "certain cell" is A1. Try this:
=IF(A1="", B1, "The cell isn't empty")
A Nested IF would contain another IF:
=IF(A1="", B1, IF(A1>100, C1, A1))
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