Miscellaneous Problems in Usage

Writers need to choose their words carefully to make sure that they are not confusing words that sound alike or have similar but slightly different meanings or that they are not using an incorrect word or expression.

Part of the fascination with space travel is the element of the unknown—the conviction that it's different from earthbound experiences. And it is.
—from "Single Room, Earth View" written by astronaut Sally Ride
The sentences correctly used it's (instead of its) and different from (instead of different than).

Negative Sentences

Do not use two negative words in the same clause. That creates an error called a double negative.
Double Negative – I can't wait no longer.
Correct – I can't wait any longer.
Correct – I can wait no longer.
The most common negative words are never, no, nobody, none, not, nothing, and nowhere. Other negative words to watch out for are barely, but, hardly, and scarcely.
Double Negative – There isn't but one cookie left.
Correct – There is only one cookie left.
Double Negative – I couldn't hardly wait.
Correct – I could hardly wait.

Common Usage Problems

Many words that have similar meanings, spellings, or sounds are confused for each other. Here are some pairs that give writers and speakers trouble:
accept / except beside / besides set / sit
affect / effect farther / further than / then
all ready / already lay / lie their / there
among / between of / have to / too
bad / badly saw / seen where / that