"That" vs. "Which"

``that'' is used to introduce restrictive clauses, meaning ones that go to define the noun being modified.

``which'' is used to introduce nonrestrictive clauses, meaning those that add additional information not necessarily required. In this context, ``which'' is always preceded by a comma.

``The computer that Jack administered crashed.''

``The computer, which Jack adminstered, crashed.''

The first sentence indicates specifically which computer crashed. The second adds additional information about the computer whose specific identity was apparently already established. Read them out loud, emphasizing the delay at the commas, and I think you'll hear the difference.

If you try to use ``that'' instead of ``which'', it sounds wrong and doesn't convey the right meaning, to my ear, at least. However, using ``which'' instead of ``that'' sounds fine. Supposedly, technically, it isn't, but it's well established and I don't think that you'll find anybody who's going to correct you on it. ``Which'' even sounds better in certain contexts, especially when ``that'' is already in use in the phrase it introduces. I think that the big thing is using the commas. That really identifies how the phrase is being used more than the introductory word, and I think that, at least in speech, most folks have that one down.