OK, for round three, there is no coherent theme. It's just about random things I feel like addressing!
Subjunctive v indicative. For those of us who've studied Latin-based (aka Romance) languages, this is not new. "¡Ojalá pudiera verte!" not "¡Ojalá podría verte!"(Span.) or "Je veux que tu prennes du café" not "Je veux que tu prends du café" (Fr.) or "Spero che tu stia bene" not "Spero che tu stai bene" (Ital.) and so on.
But what about in English? Does English have a subjunctive? Yes, but its usage is on the decline. Consider "If I was in your shoes" v "If I were in your shoes". Most people can identify the latter as sounding more correct (and they are right); but we all acknowledge that the former is becoming more and more common. However, there are certain areas where the subjunctive aids disambiguation, so I expect it to stick around for a long time in those. Consider "Was I in charge" v "Were I in charge": if you fail to use the subjunctive, it sounds like a question. We also find the subjunctive holding on in many fixed expressions, e.g. "If need be", "Truth be told", "far be it from me", etc.
The main reason it appears to be disappearing is that in so many cases, you simply can't distinguish between indicative and subjunctive in the modern form of English*. English has very simple verb conjugation, so the same conjugation is used across many different persons, numbers and even tenses. Consider the present tense indicative: only one (third person singular) is conjugated differently from the rest, e.g. I run, You run, S/he/it runs. So even when we are using subjunctive, it is often disguised because it is indistinguishable from the indicative. Example: "I require that you come to dinner." This is subjunctive, but it is conjugated no differently than if it were indicative, e.g. "You come to dinner". But try it with the third person singular and the subjunctive more obviously rears its head: "I require that he come to dinner", not "...comes to dinner".
'An'. What could be easier than the use of 'an' v 'a'? If it starts with a vowel, use 'an'; otherwise, use 'a'.
Well, two problems here. The first is a common misunderstanding about what a vowel is: vowels are sounds, not letters. The same is true of consonants. That's why linguists use the IPA (international phonetic alphabet): to understand language, you must divorce sounds from letters. It just so happens that the letter 'c' is used to represent a /s/ sound in English sometimes, but that sound is not inherent in this graphical representation called 'c'. So, applying this to our vowel issue, remember than 'an' is used before a vowel, not just a letter we usually associate with vowels. Example: we normally associated 'u' with a vowel sound. But sometimes is plays the role of what's called a 'semi-vowel', a sort of hybrid between a vowel and a consonant. When it does, as in the case of 'universe', we do not use 'an'. No one says "an universe", right?
The trickier problem is related to a very unstable phonetic element: the aspiration 'h', e.g. the first sound in 'historical'. This is a very unstable sound in languages generally, so it tends to disappear over time. That's why (in American English) 'herb' is now pronounced more like 'erb'.** (More on this later.) So when we have these cases of unstable aspirated 'h' sounds and they have become very weak, it is perfectly acceptable to use 'an'. Many people believe it isn't, because the word doesn't start with a vowel. But when the 'h' is so weak and the next sound is a vowel, it makes sense. Example: "I read an historical account of the sinking of the Lusitania." The only caveat is that the word must not only suffer from a weakly aspirated 'h' at the beginning, but must also not have a stress on the first syllable. So we do not say "an history", for although the aspiration is not very strong, the stress coming on the first syllable does accentuate it to the point that 'an' seems out of place. If you think about it, you will see this 'rule' works pretty well. Think about the following: "I rode a helicopter to an historical site to see an historian writing a history of helicopters."
Next on the list of random things I feel like addressing: hyphenation. Specifically, I want to draw attention to hyphenation as it relates to creating single concepts. For example, if you want to form a single modifier from two modifiers, you must hyphenate: "He's an easy-going fellow.***" The reason it is practical to use a hyphen here, is that without the hyphen, the two modifiers appear to apply independently to the modified noun: "He is an easy going fellow." In this case, is he easy-going or is he both easy and in the process of going? It can be confusing sometimes because there are cases in which a given phrase might be used with our without a hyphen, and the semantic gap may be a small one. My wife asked me the other day if this sentence required a hyphen: "I am a fourth-grade teacher." It does, because these two terms together ('fourth' and 'grade') constitute a single modifier for 'teacher'. But the sentence "I teach the fourth grade" does not require a hyphen as it is an adjective ('fourth') modifying a noun ('grade'). But "I teach a fourth-grade class" requires the hyphen because, again, the two modifiers combine to form a single element modifying 'class'.
'Ain't'. Believe it or not, I have no problem with this word whatsoever. When I was growing up, my mother used to correct me, saying "'Ain't' ain't in the dictionary!" In fact, it is, as well it should be. So why is it so often seen as 'incorrect'? I blame what I only half-jokingly call 'Written Language Syndrome'. WLS afflicts languages like English by deluding people into thinking that spoken language should follow written language completely and slavishly. The logic thus leads us to believe, for example, that since 'ain't' was born of strictly colloquial, verbal English, it has no place in 'proper' speech. Why shouldn't it? It is a convenient contraction, and a flexible one to boot, since it serves several combinations. I would agree that, since it evolved at the spoken colloquial level, it has no place in formal writing; but I see no reason at all to shun it at the spoken level.
*It was easier in earlier stages of English, since we had more varied conjugations back then.
**It is still pronounced with the initial /h/ sound in England, but this is tied to socio-economic class issues. 'Dropping one's haitches' has always been a clear marker of belonging to a lower socio-economic class, so educated English people who live in horror at being thought (or revealed to be) 'lower class' frantically pronounce all their initial 'haitches'. For this same reason, I would wager that most of them would object to my application of 'an' to anything that has even a wisp of an 'aitch'! 'Aitch' that a pain in the 'aitch'?
***Yes, he is...except with grammar.