3. Don't use "which" as a relative pronounFrom this article on bogus grammar errors.
The bogus idea here is that only that, never which, should be used to introduce so-called defining or restrictive clauses. For example, "The United States is one of the countries which that failed to ratify the Kyoto Protocol." One again, this is totally made up. Geoffrey Pullum, co-editor of the authoritative Cambridge Grammar of the English Language, has written, "The alleged rule has no basis. Even in edited prose, 75 percent of the instances of relative 'which' introduce 'restrictive' relatives." The culprit here seems to be the great language commentator H.W. Fowler, who popularized the notion in his 1926 book, Modern English Usage.
In fairness to Fowler, he merely speculated that if writers were to follow this custom (as he acknowledged they currently did not), "there would be much gain both in lucidity & ease." Language sticklers took that and ran with it, and this idea reigned for most of the rest of the century. Even now, it has a lot of adherents. But it still doesn't have any justification. One of the great sticklers, Jacques Barzun, advised in a 1975 book that we ought to avoid such whiches. But as Merriam-Webster's Dictionary of English Usage points out, on the very next page Barzun broke his own rule, writing, "Next is a typical situation which a practiced writer corrects 'for style' virtually by reflex action."
(My caveat is that I will still peeve if someone tells me my trad usage of a word whose meaning has changed is now wrong. My 'begs the question' is not your 'begs the question' but mine is still right.)