These rules apply primarily to formal writing. Some of them also apply to speaking and informal writing but one must recognize that some situations dictate a slightly looser application of the rules for the purpose of establishing a closeness or rapport with the audience or readers.
My first and most important piece of advice is to get a copy of The Elements of Style by W. E. Strunk and E. B. White. This very short book covers most of the common writing mistakes and provides excellent suggestions for improving one's writing. It is highly readable, which is saying a lot for a grammar book. Another interesting book is Eats, Shoots & Leaves by Lynne Truss. This is a humorous book on punctuation, which sounds boring but it was a best seller in 2004.
1. Which/that. Example: "Investment managers are constantly looking for new ideas which can increase their portfolios' returns." This is wrong. "Which" is a non-restrictive pronoun. This means that its use is not critical and would not change the meaning of the sentence if it were omitted. "That" is a restrictive pronoun. It changes or modifies the meaning of the sentence. The use of "which" nearly always requires a comma, implying that the information is not totally necessary for the sentence to be correct. Consider the following two sentences:
"The sales divisions, which the manager was responsible for, were not performing up to par."
"The sales divisions that the manager was responsible for were not performing up to par."
The first sentence implies that the manager is responsible for all sales divisions. The second sentence makes it clear that only those divisions that the manager was responsible for were not performing up to par.
Franklin D. Roosevelt, for all of his Harvard and Columbia education, was a frequent violator of this rule. Speaking about social security in 1934, he proclaimed that it was "a right which belongs to every individual." Seven years later he referred to December 7, 1941 as "a day which will live in infamy." So it wasn't just a one-time thing: he was a repeat offender.
Let me hasten to add that this mistake is commonly made in academic journals and in professional writing, or in other words, by some extremely intelligent persons who were educated at America's greatest universities. As Strunk and White recommend, good writers often go "which" hunting and eliminate the incorrect use of this word.
2. Alternative/alternate. "Alternative" implies a choice. "Alternate" implies a switching back and forth, such as every other item in a series. Consider the following sentence:
"In preparing for battle, the general developed an alternate plan."
This is wrong. The general almost surely does not expect to switch back and forth between different plans during the battle. What he devised was an alternative plan. The use of "alternative" instead of "alternate" is the more common mistake, perhaps because the word is slightly shorter and economy is nearly always recommended in writing.
3. Overuse of parenthetical expressions. Although there are legitimate uses at times, in general parentheses should be avoided. Consider the following sentence: "The focus of this paper is on the use of quantitative techniques (such as statistics, mathematics) to solve business problems." Parentheses often imply that the writer is turning aside to the reader the way a speaker might do an aside to an audience. A more appropriate sentence here is "The focus ... quantitative techniques, such as statistics and mathematics, to solve ..."
There is little room for parentheses in formal writing. On occasion some are necessary such as "Sales are expected to increase (decrease) if we lower (raise) prices." Though some people do not like this usage, it is a convenient way to use parentheses to reduce the number of words and it is perfectly clear to any marginally literate reader what it means.
Another acceptable use is for example, "The net present value (NPV) is the best measure of the attractiveness of a capital investment project." Such usage gives the writer the freedom to use the letters "NPV" later in the document instead of "Net Present Value."
4. The improper use of "only". For example, "We only have room for four people in this taxi" seems rather harmless and is probably acceptable in speaking but not in writing. Consider the following two sentences:
"I eat only in the kitchen."
"I only eat in the kitchen."
The first implies that I never eat anywhere else such as the dining room, patio, etc. The second implies that I do nothing but eat in the kitchen. I never prepare food, wash dishes or do anything else in the kitchen.
This is easy to remember but I have also heard it expressed by changing the words "eat" to "have sex" and "kitchen" to "bedroom," which you may even find easier and more interesting to remember.
5. Incorrect use of "it's" and "its". This one is incredibly simple. "It's" is short for "it is." "Its" is a possessive pronoun. Whenever you use the word "it's" read it out as "it is" and see if it makes sense. This leads us to the next item.
6. Use of contractions. Contractions such as "it's" should be avoided in formal writing. Write out the whole words "it is". In informal writing, however, contractions are not only preferred, they are pretty much mandatory. In fact, I would say contractions are almost the primary difference between formal and informal writing. There is a time and a place for both.
7. Active/passive sentences. Excuse me, Mr. or Ms. Grammar but I beg to differ on this one. One of the most common recommendations for good writing is to use active instead of passive sentences. For example, "The surgeon removed the tumor" is an active sentence. "The tumor was removed by the surgeon" is a passive sentence. This is fine in many cases but when describing what you are doing or have done, the passive voice usually sounds better. There is nothing worse than having to write repeatedly that "I" or "we" are doing something or did something. This directs the focus of the writing toward the writer and you will be criticized for having done that. When describing what you have done or are doing, use the passive voice. The surgeon writing an article for a medical journal would, therefore, say "The tumor was removed," which is a passive sentence, rather than "I removed the tumor," which is active.
Naturally in some forms of writing the active voice is better. If you are writing an autobiography or describing something you have done in informal writing, the active voice is usually preferred. This is not usually the case in formal writing such as a report or research paper.
8. Underlining. Underlining is a throwback to the days when typewriters were used. The standard typewriter could not do italics. With the advent of typewriters with interchangeable fonts and, of course, word processors, the underline has fortunately gone by the wayside. Titles of books, periodicals, movies, etc. that were formerly underlined should be put in italics.
9. But. This word is often used in a manner represented by the following context: "He could not help but do worse." The correct usage is "He could not help doing worse."
10. Data/Datum. Data is a plural word. The singular form is datum. So for example, "The data are collected" is the correct form in spite of how strange it may sound. The word "datum" is rarely used, owing to the difficulty of ever encountering a minute piece of information such as a datum. Almost every encounter will be with more than one datum; hence "data" is the word nearly always used.
Last updated: January 9, 2011