When you use a word incorrectly, you hurt your credibility. Even if the mistake is an understandable typographical error, you lose the reader for a moment, and you send the subtle message that you don’t know or don’t care what the correct word is. Here are some common word mistakes in two categories.

Usage mistakes

ensure / insure
To “ensure” is to see to it that something happens: “Please ensure that the motion is filed.” But “insure” means to take a financial risk requiring payment for loss: “Our company cannot insure your home against flood damage.” Using “insure” when you mean “ensure” is a common mistake.

forego / forgo
Surely one of the most common spelling errors in legal writing. Perhaps it’s because “forgo,” which means “to do without,” just looks odd. But it’s right. To “forego” is to go before.

i.e. / e.g.
I don’t use these Latin abbreviations for “namely” or “that is” (i.e.) and “for example” (e.g.). I’m generally against Latin in legal writing. But if you use them, don’t confuse them. And don’t italicize; it’s not necessary. Bryan A. Garner, Garner’s Modern American Usage 289, 421-22 (Oxford U. Press 2003).

that / which
When you have a number of things, and you need to restrict what you’re writing about to one of those things, you need “that” and no comma. “That” is restrictive:

• The lawnmower that is broken is in the garage. (I have more than one lawnmower, and I need to restrict this sentence to the one that is broken.)

But if you’re writing about one thing, and you want to add some extra or clarifying information about that thing, you need a comma and “which.” “Which” is nonrestrictive:

• The lawnmower, which is broken, is in the garage. (I have only one lawnmower, and it is broken.)

This might help. The difference between “that” with no comma and “which” with a comma is the same as the difference between these two sentences:

• The security guard who attacked me was wearing sunglasses.

(There is more than one security guard, and I’m writing about the one who attacked me—same as the restrictive “that.”)

• The security guard, who attacked me, was wearing sunglasses.

(There is only one security guard and, by the way, he attacked me—same as the nonrestrictive “which” with a comma.)
 
Mastering the difference between “that” and “which” can be difficult. If you don’t have it down yet, keep studying and default to “that.” You’ll be right most of the time.

Typos

than / then
We all know the difference between these words, but when we’re typing, our fingers can slip. Search your document for them and double-check.

its / it’s
Law school graduates generally know the difference between “it’s,” a contraction for “it is,” and “its,” a possessive pronoun. So this mistake is usually a typing error. But if you struggle because “it’s,” with the apostrophe, seems like a possessive, then remember that “its” is like the other possessive pronouns that don’t take an apostrophe: hers, his, theirs.

trial / trail
A common mistake that the spell-checker won’t catch. Although it is sometimes funny to see references to the “trail judge” and a “trail lawyer,” it’s funny only if you’re not the author. Why not do a search for “trail” in every court document you write?
 
For further guidance, see Garner's Modern American Usage and Fowler's Modern English Usage .