A nested If-Then is used when you want to test a second condition based on the result of the first.
If (x = 5) Then
If (y = 15) Then
z = 25
Else
z = 0
End If
End If
This is NOT the same as:
If (x = 5) And (y = 15) Then
z = 25
Else
z = 0
End If
Nested If-Then code blocks can go many levels deep. It's best to avoid too much nesting, as the code can become confusing.