34. Obey these rules before you obey grammarians, who say things like "Don't split infinitives" or "Don't begin sentences with And or But" and "Don't end a sentence with a preposition."

Real grammarians, i.e. linguists who study the grammar of English as it is, teach us that these aren't actually rules of grammar anyway, so much as prescriptions that were made up out of whole cloth for various reasons and that never had much to do with the way English was spoken or written. Here, for example, is an index of postings on Language Log (a group blog run by several professional linguists) about the split-infinitive issue. (The well-known story of this silly prescription was that it was decided in the 18th century that, since you can't split infinitives in Latin [Latin infinitives are a single word], you shouldn't split them in English either.)

Relatedly, the passive in English has a bad reputation that is not very well deserved. See here for a full explanation by the author of the Cambridge Grammar of the English Language.

You'd think this was just so much nitpicking -- and to some extent it is -- but understanding these issues fully can help you make better rhetorical use of English. This is particularly true of the passive -- the article I linked above explains how passive and active versions of the same clause help us place emphasis in a sentence exactly where it will do us the most good. (As such, I think the strongest version of your point 13 that I could endorse would be "Understand clearly the difference between active and passive, and choose between them advisedly.")

One more point which I raise not least because it's a stunningly entertaining read: the same author's (Geoff Pullum's) "The Land of the Free and The Elements of Style" (PDF), an utter demolition of the grammar advice given in Strunk and White's book. This is NOT to say that S&W's stylistic advice should be thrown out as well, but Pullum certainly establishes that (a) they have absolutely no idea what they're talking about when grammar is concerned, and that (b) they follow almost none of their own grammatical or stylistic prescriptions, so the whole thing should be taken with a grain of salt. Read Pullum's article if you enjoy a well-deserved poison pen book review and would like to learn a few things about English grammar in the process.