02/08/2014 01:50 pm ET Updated Apr 10, 2014

Brushing Up on Grammar: Misunderstood Words and Troublesome Prefixes

A good friend of mine asked me the other day if it was correct to say, "taken back" as an expression of surprise. The answer is "no"; the correct phase is "taken aback." That may sound funny to some people, but it is correct.

This caused me to think about another word and some prefixes. So I decided to write about "aback," "unkempt," and how to use "in" or "im" or "il" or "ir" as a prefix with adjectives. Let's take them one by one.

Misunderstood Words: "Aback" and "Unkempt"

"Aback," an adverb, dating from AD 1,100 and coming to us from Middle English, has three historic definitions: (1) "back or backward," now considered archaic and no longer used in English; (2) "in a position to catch the wind upon the forward surface (as of a sail)" and used primarily by people of the sailing world: and (3) "by surprise" or "unawares," the definition currently used by the majority of people.

Using "taken aback" as an expression of surprise or shock is the correct usage of "aback" in today's English. It may seem unusual or out of place because it is, in fact, an idiom -- that is, an expression that is peculiar to itself grammatically and in meaning.

Here are some examples of its correct use in a sentence.
"I was taken aback when I got his unexpected letter."
"I was taken aback when I fully understand the minister's sermon."
"You may be taken aback to know that your problem isn't a big drawback after all."

The elderly in some parts of Scotland tend to use "aback" the same as "ago," for example, "Eight days aback." But that usage is unique and is going out of style. The correct English usage of "aback" for most of us today is as an expression of surprise.

"Unkempt," an adjective, is often confused with "unkept." They have entirely different meanings and should not be used interchangeably.

Unkempt means "uncombed," and primarily refers to uncombed hair, for example:
"Your hair looks unkempt when you roll out of bed in the morning."
"When you are outside without your hat in the wind, you hair looks unkempt"

There is some occasional usage, especially in literature, of unkempt to refer to anything that is deficient in order or neatness -- for example, unkempt gardens, an unkempt rambling path, or unkempt villages -- but unkept is the appropriate word to be used in these instances.

"Unkempt" comes to us from Middle English (12th-15th centuries) and is traced to "un" (not) and "kemben" (to comb). "Unkempt" should be used in referring to uncombed hair and "unkept" in referring to something that is neglected.

Troublesome Prefixes with adjectives: "in," "im," "il," and "ir"

A few weeks ago another friend told me that he could never remember whether "imput" or input" is correct. I told him that there is no such word as "imput," but I added that there is a definite grammatical formula that explains how to use "in," "im," "il," and "ir" as prefixes with adjectives. As I thought about our conversation, I remembered how many times my students expressed frustration over not understanding the correct usage of these prefixes. So let's clear this up.

Just as a reminder, a prefix is a letter or letters attached to the beginning of a word, thus producing a new word that has been derived from the word the prefix is attached to and has a different or expanded meaning from the original word, many times producing a word with the exact opposite meaning as the original meaning.

When used as a prefix with an adjective, "in," "im," "il," and "ir" are related and have exactly the same meaning. They all can be traced back to the Old English period of grammar (7th to 12th centuries) and have come to us from the Old English to Latin to Anglo-French to Middle English to the present. They all have the same origin; they come from the Old English word "un," translated into modern English as "not" or "non."

Now, here's the formula for knowing whether to use "in" or "im" or "il" or "ir" as a prefix with an adjective. In most instances, English grammar calls for "il" to be used before words beginning with "l" (for example, illiterate and illogical); for "im" to be used with words beginning with "b" (imbalance), beginning with "m" (immoral and immortal) or beginning with "p" (impractical and imprecise); "ir" to be used with words beginning with "r" (irreplaceable and irresistible); and "in" to be used with other sounds (for example, inapplicable, inconsiderate, inconsolable).

The words in parenthesis are meant only as examples, not as all-inclusive lists. It is interesting to look in a dictionary in the sections that have "in," "im," "il," and "ir" to find the many, many examples of how these four prefixes are used with adjectives.

These four prefixes can also be used with nouns, but then they take on a different meaning, so that is a subject for another day.

Hopefully, you will find these comments helpful in brushing up on English grammar.

All definitions are from Merriam-Webster's Collegiate Dictionary, Eleventh Edition, 16th Printing, 2012.