I often see a single word that should be two words. Here are some examples.
Setup and set up
“Setup” is a noun, usually meaning an arrangement. “The setup worked well.”
“Set up” is a verb, meaning to assemble or put in readiness. “She set up the display.”
Logon and log on
“Logon” is a noun, describing the “procedure used to get access to an operating system or application, usually in a remote computer” (Whatis.com). Logon can also be used as an adjective, as in the “logon procedure.”
“Log on” is a verb. “I need to log on to my computer.”
The same distinctions apply to “login” and “log in.”
Alot and a lot
“Alot” is not a word. Use “a lot.” Fortunately, the spell checker in Word flags this common error.
Everyday and every day
“Everyday” is an adjective meaning ordinary or typical. “The store offers everyday values.”
“Every day” is an adverb referring to something that occurs each day. “The store offers values every day.”
Backup and back up
“Backup” is a noun, most often referring to a computer file that has been saved. “My backup was corrupted.” “Backup” can also be an adjective. “It’s important to have a backup plan.”
“Back up” is a verb, referring to the action of moving backwards (e.g., a car) or of saving an electronic file to another location. “I am going to back up my computer right now.”
Follow-up and follow up
“Follow-up” can be used as a noun, meaning “the act of following up.” For example, “the follow-up was effective.” “Follow-up” can also be used as an adjective. “Follow-up actions are essential.”
“Follow up” is a verb, indicating action. “I will follow up with you tomorrow.”
Lookup and look up
“Lookup” is a noun, meaning “the process or instance of looking something up; especially the process of matching by computer the words of a text with material stored in memory” (Merriam-Webster Online Dictionary).
“Look up” is a verb. “I’m going to look up the address.”
The same distinction holds true for “hookup” and “hook up.” The single word is the noun.
Anyway and any way
“Anyway” means anyhow. “I am going to go to the party anyway.”
“Any way” means “a way.” “Is there any way you could drive me to the party?”
Awhile and a while
Both “awhile” and “a while” mean “for a short time.”
Use the two words if they follow a preposition. “He wanted to nap for a while.”
Otherwise, use the single word. “She plans to be gone awhile.”