Select...Case Statements

I'm trying to ensure the data in a certain column (H) can't be changed before the data in another (F) has been updated. I started with an If statements but that meant one couldn't update anything in column H at all. I've started to work on a Select Case statement instead, to evaluate whether the user is trying to change the value of a cell in column H, undoing it and changing the active cell to column F of hte same row. However I can't seem to get my head around how to make it so that if one HAS updated column F that changing column H is also possible.

I've got this far:

Public Sub Change_Event(ByVal Target As Range)

Select Case Target.Column

Case Is = Target.Column = 6

Target.Cells.Offset(0, 2).ClearContents

Case Is = Target.Column = 8 And Target.Cells <> 0

MsgBox "Update Date Last Seen Date before changing review date " & Range("AC1").Value, _
vbCritical, "Error Message"

Application.EnableEvents = False

Application.Undo

Application.EnableEvents = True

Target.Offset(0, -2).Select

Case Else

Exit Sub

End Select

End Sub

but nothing happens.

Any help appreciated

Thanks