For example, I have an issue with the misuse of “fewer” (to be used when you can count things, like doughnuts) and “less” (for things you measure, like flour or rain). It is true that you can use fewer cups of flour or less flour. And you can put fewer dollar bills on the table or have less money than you had last month.
I’m happy to announce, on the other hand, that it hasn’t mattered for years—decades, in fact—whether our sentences end with prepositions. It’s far more effective to say, “That’s the pool the little boy was sitting in,” than to say, “That is the pool in which the little boy was sitting.”
Which brings us to “which” and “that”—and also when to use fragments. In 1980, I had one sentence fragment in my 300-page Ph.D. dissertation. I was using it for effect, but they told me to take it out. (Although I didn’t.) A sentence fragment to create a change of pace or add a colloquial feel usually is OK. A sentence fragment caused by the inability to recognize or hear the logic of subjects, verbs, and dependent vs. independent clauses is not OK. It is not OK to write, “The Democrats exhausted after a long campaign.”
Back to the use of “which” and “that,” I confess that I did not have a clear understanding of the relevant context for many years. One must be able to identify restrictive uses vs. non-restrictive uses. Then I finally got it. A comma usually comes before “which” because it introduces a non-restrictive idea, kind of an after-thought: “The man raised his voice at her, which was entirely uncalled for.” A comma usually doesn’t come before “that” because the idea it precedes is restrictive—necessary to the completion of the idea: “Morton’s dog is the one that bit her.” However, sometimes it just feels right, especially in creative writing, to use “which,” with no comma before it, instead of “that.” We need to cut down on our use of “that” anyway, so sometimes switching to “which” is a solution.
In the paragraph above, I used the words “usually is OK.” Many or most people would write or say “is usually OK.” However, I assimilated a rule once that proscribes the placement of an adverb between a verb and a complement or between a helping verb and a participle (“he had run the course easily” is better than “he had easily run the course”). Because I have internalized that rule, I rewrite a sentence if I realize I have interrupted a verb phrase with a modifier.
But you know what? In 2016, in most contexts, it doesn’t really matter.