[QUOTE=SmackFu]
Is affect/effect really a necessary distinction? Are there sentences where you could use both, and which one you choose changes the meaning?
[/QUOTE]
No, they are two distinct words with separate applications.
Pedants who complain about common usage shifting meanings, often because non-professionals use words “incorrectly”, tend also to think that language descriptivists say that anything goes and that anything anybody does must be right.
It’s nowhere near that simple. First, there are numerous levels to language. Grammar, spelling, meanings, usage. Grammar tends to stay fairly fixed and changes only slowly over time. Spelling was mostly fixed in the 19th century after dictionaries and spelling books became common and public education made the teaching of “rules” a necessary part of learning.
Meaning and usage are far more slippery. Virtually every common English word has accrued additional - often many additional - meanings over the centuries. If you ever see someone in a language thread protest that a word “originally” meant something, they have lost the argument. (If you ever see someone complain you can’t use a plural like “they” with a singular like “someone”, they have lost the argument.) Complaints about the breakdown of the English language can be found from literally any year in the past several hundred. They are all nonsense. The English language is better than it ever could have been imagined in the past.
So what about words like affect and effect or the use of apostrophes in plurals. Do we have to accept them and approve of them?
What perplexes the pedants is that the answer is still no. The English language does not have an official body that monitors and attempts to control it. What it does have are large numbers of good writers who set boundaries for what good language is.
It is trivially easy to find good writers - i.e., professionals in serious work: newspapers, magazines, books, journals - who use enormity to mean large in size. That is probably the most common usage in print today.
You will almost never see a good writer use affect when effect is meant. You will virtually never see a good writer use an apostrophe in a plural.
The English language can be divided into use by good writers and use by poor writers. It isn’t difficult to tell which side of the divide most people are on. (The border is fuzzy, as borders always are.) Not every good writer writes flawless prose, whatever flawless might mean. Not every poor writer produces illiterate sludge every sentence. The totality of the writing - the context, again - normally makes the distinction easy for everyone to see. Even poor writers can tell what good writing is, whether they can do it or not.
The American Heritage dictionary made news when it first appeared by polling a couple of hundred writers on their views of whether certain terms or usages were acceptable. The real lesson of the results didn’t lie in what percent approved what, but that virtually every one of them had a different list of acceptability. When they were re-polled for later editions, most usages became more accepted (a few went the other way). No two good writers, experienced professional authorities, will agree on what is acceptable or what is proper, even as a time-limited snapshot of the language. Moreover, the language is changing faster than at any recorded period.
There’s nothing wrong with asking about usages or what usage to use or how to improve one’s writing by tilting it toward what is considered good writing. The problem comes only when people want an answer, one definitive “do it this way” answer of the sort that lazy elementary school teachers give. Only pedants will provide definitive answers, which is why pedants are so scorned by those who better understand how English works.
You can no more give an answer to this question than to the question, “how do you hit a golf ball”? You hit a golf ball by having a good fundamental swing and adapting it to the situation you find yourself in. You write English the same way. Finesse and feel, not rules. That’s not the answer that people want to hear, but it’s the only one available.