My Grammar and I (Or Should That Be 'Me'?): Old-School Ways to Sharpen Your English

Non-restrictive: none of the dogs
recognized the thief and all of them
barked.

---

Or
I cut down all the trees
which were evergreen.
Restrictive: some of them were
deciduous and I left them alone.
I cut down all the trees,
which were evergreen.
Non-restrictive: I’ve destroyed the
entire forest and, by the way, all the
trees were evergreen.
In both cases, the which in the restrictive clause could be substituted by that , but
this  wouldn’t  work  in  the  non-restrictive  clauses.  Non-restrictive  clauses  are
introduced by the relative pronouns who, whom, whose and which , never by that
.
Grammarians  are  divided  over  whether  which  or  that  should  be  used  in
restrictive clauses. While  researching this book,  we came across  all of the
following in apparently respectable sources:
‘Many  grammarians  insist  on  a  distinction  without  any  historical
justification.’
‘Which and that are equally acceptable in restrictive relative clauses; that is
perhaps the less formal of the two.’
‘Don’t mix which clauses with that clauses.’
It  boils  down  to  this:  if  you  can  tell  what  is  being  discussed  without  the
which  or  that  clause,  use  which  ;  if  you  can’t,  use  that  .  Or,  as  a  rule  of
thumb, if the phrase needs a comma, you probably mean which .