awhile versus a while
This one is tricky, in that awhile is an adverb, and a while is a noun phrase meaning “length of time.”
She plans to be in England awhile.
She plans to stay in England for a while.
breakup versus break up
Breakup is a noun meaning the dissolving of a relationship. Break up is the verb form.
The breakup hurt Missy more than she had thought.
Riot police could not break up the protests in the downtown streets.
cannot versus can not
Although technically both forms may be used, cannot is more accepted in modern language.
I cannot understand why the Packers won last night.