We now move down to the most microscopic level of writing: the individual words themselves. Usage has to do with the meanings of words and the rules for when they should -- and shouldn't -- be used. What follows is a non-comprehensive compendium. I should note that these "rules" are among the ones that people like Pinker and Pullum make fun of, so you may want to take them as more pedantic than absolutely necessary.

That/which.

Many people believe that in American English one should use the relative pronoun "that" to introduce restrictive clauses and "which" to introduce non-restrictive clauses. What does that mean? A restrictive clause is one that restricts a noun to a particular instance or class. For example: "The book that was on the table is mine." Contrast this with the non-restrictive variant: "The book, which was on the table, is mine." (This latter type of clause is also called a descriptive clause.) Notice that the non-restrictive clause is set off by commas.

Due to.

The expression "due to" should be used only to attribute a state of affairs, not to indicate causality. For instance, "this equation is due to Einstein" is OK; but "the volcano erupted due to underground lava pressure" is wrong. In 90 per cent of the cases in which people use "due to," they should have used "because of." Incidentally, I know of absolutely no instance in which the horrendous expression "due to the fact that" should be used. Get rid of it.  There is nothing wrong with the word "because."

Hopefully.

Although English permits many adverbs to be used in dangling, free-floating form, current opinion has singled out "hopefully" as a major no-no. Never use this expression to mean "I hope" or "it is to be hoped." You can use it only as an ordinary adverb modifying a specific verb. "Hopefully this megatron space-prober will work on the first try, said Tom" is out; but "I know this megatron space-prober will work on the first try, Tom said hopefully" is OK. In the first case "hopefully" just floats around the sentence trying to convey guarded optimism; in the second case it modifies the verb "said." Other free-floaters like "unfortunately" and "happily" have passed safely into accepted usage, and worrying about "hopefully" is no doubt another losing battle.

Fortuitously.

Fortuitously means simply accidentally or by chance. It does not mean "fortunately" or "luckily." It does not even mean "by lucky chance." Same goes for the adjective form, of course.

Currently/presently/momentarily.

If you want to convey the idea that something is happening now, use currently. ("The theater is currently showing three Marx Brothers flicks.") Or, better yet, just say "now." I object to the use of presently as a synonym for "currently" or "now" -- although I'm probably on the losing side of this one too. Presently means soon or in a little while. ("Dinner will be served presently.") "Momentarily" means for a moment not in a moment. When the captain says that the aircraft "will be in the air momentarily," you'd better hope that his usage is bad.

Continuously/continually.

The adverb continuously must refer (perhaps metaphorically) to a process that is continuous in essentially the mathematical sense -- there are no gaps or holes. ("His routine was a continuous progression of one-liners.") If you want to refer to repeated events that are discrete or intermittent, use continually. ("He remonstrated continually [not continuously] on the virtues of sacerdotal celibacy.") Again, same goes for the adjective.

Comprise.

To comprise means to include. If you can't substitute "include" for "comprise" in your sentence, you're misusing the word. More formally, "comprise" can be used only in reference to a proper subset of a specified set. That is, "New England comprises Massachusetts, Connecticut and Rhode Island"; but "New England is composed (or made up) of six states." The whole comprises the parts, not the other way around. And the ubiquitous expression "comprised of" is always wrong.

Different from/differently than.

The adjective form is always followed by "from," the adverb form by "than." Thus: "I came up with an answer different from (not than) his. That's because I approached the problem differently than he did."

Between/among.

Don't forget that "between" should be used only when the connection involves precisely two things; "among" should be used for three or more. "The choice among the twelve contestants resolved itself into a choice between the two leading candidates." Apparently, the exception is when the connection among multiple items is "dyadic," that is, two at a time. "To fulfill the requirement you must take two courses, and you can choose between Economics 101, 102, or 103."

Less than/fewer than.

As in the case of continuously versus continually, the desideratum here is continuity versus discreteness. Use "less than" when the comparison involves a continuously measurable quantity ("less than three inches long") or when the number involved is large ("less than a billion stars"); use "fewer than" when the numbers involved are small and discrete ("fewer than ten people per lifeboat"). The sign over the express-checkout lane in the supermarket is wrong.

Convince/persuade.

You can convince someone of something, but you can't convince someone to do something. You persuade that person to do it.

Ensure and enquire.

I've never seen anything written about this, but my own rule is to use ensure when I have in mind the loose or metaphoric meaning ("He stayed behind to ensure that everyone was safe") and to use insure only when I mean the technical activity of providing insurance ("This procedure effectively insured the farmers against crop losses"). Similarly, I use "enquire" for the general activity of asking, and reserve "inquire" for the kind of penetrating research that scientists and detectives do.

Unique.

There are some adjectives and participles (pregnant and dead come to mind) that can't be qualified (that can't be modified with an adverb). You can't be partially dead (in a non-metaphoric sense) or kind of pregnant. Unique is another example: it means "one of a kind," and to say "rather unique" is illogical and considered unacceptable in written English. There are other such adjectives, so be on the lookout.

Nouns as verbs.

It is a long-established process in English to transform nouns into verbs (a pitcher pitches, a cook cooks, etc.). Such transformations can add immediacy to the language; but carried too far, they can rob the language of elegance and give it a boring bureaucratic tone. A gentleman establishes priorities; a bureaucrat prioritizes. The rule should be to transform the noun to a verb if the transformation is a fresh metaphor, but to avoid such noun-verbs when they sound like bureaucratic buzzwords. Received opinion would draw the line, for example, at such words as author, critique, and host. One writes (not authors) a book; criticizes (not critiques) an essay; and plays host at (not hosts) a party. It could be worse: people are now talking of "gifting" one another at Christmas.

He, he or she, s/he, they.

There is a rule from time immemorial in English that the pronoun "he" can be used in some circumstances as a generic or androgynous pronoun -- one that refers to both men and women. It has become abundantly clear in the last few decades that this rule has passed into history. Whatever you may think of political correctness, it is now considered Neanderthal among the intelligentsia not to say "he or she" and, more generally, to arrange one's prose for greater androgyny. Use expressions like "one" or "the economic agent." Tellingly, perhaps, people seem increasingly to resolve the problem by using just "she," presumably on the grounds that no one could possibly fault you for discriminating against men. Under no circumstance, however, will I -- or should any human being of either sex -- use the formula "s/he." It's something I would expect to see in regulations promulgated by the Ministry of Political Correctness.  I should also note that one increasingly sees "they" as a solution to the problem, with "they" treated as an androgynous singular pronoun not as a plural pronoun.  Like it or not, this almost certainly the future.