That vs Which

Jack Lynch:

• If you are restricting something by distinguishing it from a larger class of which it is a member, use **“that”**:

       I chose the girl that has the sweetest smile.
  

Note that this implies there were other girls.

• When the general class is unrestricted, then “which” is appropriate:

       He chose two girls, which didn’t turn out quite well.
  

Note that this does not imply there were other girls.

• “which” is normally preceded by a comma, but “that” is not.