Overwriting options

I must have inadvertently typed a shortcut so now when I overwrite it erases the source I've copied in the target segment automatically. This means I can't edit the middle of a sentence without having to rewrite/reformulate the entire sentence!  What would be the shortcut to invert this function? 

Also, I am working on TRADOS in Windows Parallel on a Mac with a Mac keyboard... Any suggestions would be welcome! Thanks