1) ALOT / A LOT
Perhaps this common spelling error began because there does exist in English a word spelled “allot” which is a verb meaning to apportion or grant. The correct form, with “a” and “lot” separated by a space is perhaps not often encountered in print because formal writers usually use other expressions such as “a great deal,” “often,” etc. If you can’t remember the rule, just remind yourself that just as you wouldn’t write “alittle” you shouldn’t write “alot.”
2) ALL READY/ALREADY
“All ready” is a phrase meaning “completely prepared,” as in “As soon as I put my coat on, I’ll be all ready.” “Already,” however, is an adverb used to describe something that has happened before a certain time, as in “What do you mean you’d rather stay home? I’ve already got my coat on.”
3) ALRIGHT / ALL RIGHT
The correct form of this phrase has become so rare in the popular press that many readers have probably never noticed that it is actually two words. But if you want to avoid irritating traditionalists you’d better tell them that you feel “all right” rather than “alright.”
4) ALTAR/ALTER
An altar is that platform at the front of a church or in a temple; to alter something is to change it.
5) ALTERIOR /ULTERIOR
When you have a concealed reason for doing something, it’s an ulterior motive.