Remember this to remember the difference : ‘that defines, which describes’. ‘that’ introduces an ESSENTIAL DEFINITION of the subject, while ‘which’ introduces an OPTIONAL DESCRIPTION of it. The following two sentences illustrate this nicely: He stopped the first car that was driven by a woman. He stopped the first car, which was driven by a woman. First sentence means that he did not stop any car that was not driven by a woman. He stopped the first car which he found was driven by a woman Second sentence means that he stopped the first car that he came across. Then it gives an optional information that this particular car was driven by a woman :) In the first sentence, ‘that’ introduces an essential definition. Without the rest of the sentence, the meaning is completely different. But in the second, ‘which’ introduces a description, without which the sentence would still have the same meaning. Note also that ‘which’ always follows a comma, while ‘that’ runs straight on. The comma denotes a pause in speech, as you can confirm by reading out the two examples above. So you can always say your sentence out loud, see if a pause is needed, and make your selection on that basis. (If you pause, there’s a comma and therefore it’s ‘which’.) More info here :) http://www.betterwritingskills.com/tip-w022.html

- "The lawnmower _that_ is in the garage needs sharpening." We have more than one lawnmower. Only the one in the garage needs sharpening. - "The lawnmower, _which_ is in the garage, needs sharpening." We have only one lawnmower. It's in the garage and needs sharpening. - "The statue _that_ stands in the hall is on loan from the museum." A number of statues are in the building. Only the one in the hall is on loan from the museum. - "The statue, _which_ stands in the hall, is on loan from the museum." Only one statue is under discussion. It is on loan from the museum and happens to be in the hall. http://web.ku.edu/~edit/which.html