[Gimp-user] Question about Selections in Gimp

[Alice Meade <alice meade rcn com>]

When using GIMP, I make lots of selections.  Usually, the "Mode" is "Replace current selection" - which is 
what I always want it to be.  But lots of times, the Mode switches on me to "Subtract From Current 
Selection".  Yikes!  I did not want this!  It seems to do this on it's own when you have a selection and you 
press the Command button (on a Mac).  Why does it do this?????  What is the keyboard short cut to get it back 
to "Replace Current Selection"?  I cannot figure this out.

Thanks!