Style and Usage
Strunk's original (1918) Principles of Composition included:
Omit needless words; Exercise
Use the active voice; Exercise
Put statements in positive form: Exercise
Keep related words together; Exercise
Express co-ordinate ideas in similar form
Avoid a succession of loose sentences; Exercise
Place the emphatic words of a sentence at the end
A participial phrase at the beginning of a sentence must refer to the grammatical subject; Exercise
In his 1959 and subsequent editions of the book, E.B. White added a Principle of Composition
Use definite, specific, concrete language
and a number of stylistic "reminders":
- Place yourself in the background
- Write in a way that comes naturally
- Work from a suitable design
- Write with nouns and verbs
- Revise and rewrite
- Do not overwrite
- Do not overstate
- Avoid the use of qualifiers
- Do not affect a breezy manner
- Use orthodox spelling
- Do not explain too much*
- Do not construct awkward adverbs
- Make sure the reader knows who is speaking
- Avoid fancy words
- Do not use dialect unless your ear is good
- Be clear
- Do not inject opinion**
- Use figures of speech sparingly
- Do not take shortcuts at the cost of clarity
- Avoid foreign languages
- Prefer the standard to the offbeat
*Not to be confused with 16) Be clear, which is by far the most important of these 21 rules. White is referring to things like "he said consolingly," which actually fall more into the category of "needless words" (Strunk's first Principle of Composition). More generally, where clarity is at stake, I would say that you cannot explain too much, just as you cannot be "too clear." Much more often, the problem is that things are not explained that should be, which is a much worse problem then a few needless words. When in doubt about how much to say about a particular point, it is better to err on the side of clarity, rather than conciseness.
**Don't "inject" it means don't throw it in unnecessarily or irrelevantly. Expressing your opinion in a piece of writing meant to express your opinion is quite a different thing. The advice would still apply, though, since you do not want to include in one essay your opinions on subjects that have nothing to do with that essay. Your thesis is, of course, your opinion, and the whole point of writing the essay is to present it as effectively as possible.
With those two caveats, White's additions are largely self-explanatory. (The revised editions are not available online, as far as I know.)
In addition, modern compositon textbooks warn us (i.e., primarily native speakers) about:
poor word choice or diction: clichés, jargon, pretentious diction, euphemisms, false connotations, inappropriate tone, disturbing sound patterns
poor syntax or sentence structure: fragments (incomplete sentences), choppy sentences, confusing shifts, faulty subordination, mixed constructions, split constructions
poor flow of ideas: unclear pronoun reference, illogical or weak transitions, faulty logic