It's easy to do when you're in a hurry and rely on spell-check to catch your mistakes. But you probably realized that that does not always work the last time you typed "to" when you meant to type "too." As long as the spelling is correct, your computer is happy. Proper usage is your responsibility.
Do you remember when, somewhere around fourth or fifth grade, your teacher introduced you to the concept of homonyms—words that sound alike and/or are spelled alike but have different meanings? Sometimes you grab the wrong word from your memory bank because it truly is a homonym; other times, while the words you're mixing up are not actual homonyms, they have a close enough connection to be confusing. On that note, lettuce (oops, I mean, "let us") take a look at some commonly confused word pears—eh, "word pairs," that is.
Are you clear on your contractions?
Who's is a contraction of the words "who is" or "who has."
Whose is used to ask a question such as "Whose dog is this?"
Its without the apostrophe indicates possession by an inanimate object, as in "That tire has lost all of its air."
It's is a contraction of the words "it is" or "it has."
You're is a contraction of the words "you are."
Your is the second-person possessive determiner used in, for example, "Is this your work?"
Here are a few pure homonyms that are sure to make most people stop and proofread their work:
Stationary describes something that is not moving or not changing.
Stationery refers to paper and/or other writing supplies.
Shear refers to the action of cutting the wool off of a sheep.
Sheer, which is more commonly used, refers to something that is very thin.
Principal means first in order of importance.
Principle, on the other hand, refers to a doctrine, law, or belief.
Grisly means "causing horror or revulsion."
Grizzly refers to the (grizzly) bear's white-tipped fur.
Discreet describes someone who is careful not to attract attention or offend.
Discrete means "separate" or "distinct."
Council refers to an administrative or advisory group.
Counsel refers to advice or guidance.
These last two are close enough to be confusing, although they are not true homonyms:
Ordinance refers to an authoritative order.
Ordnance refers to guns or munitions.
Loath means "reluctant" or "unwilling."
Loathe means "to dislike greatly."
Mastering the sound-alike words is just the tip of the iceberg when it comes to writing right. And remember, there is no shame in owning a dictionary; if you are really in a hurry, try the "thesaurus" button right next to spell-check.