Fortunately the main rule is pretty easy, once you separate the verbs from the adjectives or nouns: Verb phrases are always separate words.
Let's see how that works with some examples from Hawaii Five-0.
"Call Chin and see if he can run a search on whether our suspect was here to check in," Steve said, looking around the hotel lobby.In the first example, Steve is referring to the suspect taking an action: checking in, a common verb phrase. Speaking of checking in, see what happens when another form of the verb is used. You'd never write "the suspect was checkining," or "he checkined yesterday." Whether it's the infinitive or conjugated, the verb part of the verb phrase stays separate.
Danny responded, "Why don't I just ask at the check-in counter?"
In the second example, we see the corollary to the main rule: Verb phrases that have evolved to an adjective or noun form are either hyphenated or become a single word. Danny is going to the counter that is described by the adjective check-in, to differentiate it from the concierge counter or the bellhop counter.
Here's another example:
Kono pointed at the flyer taped to the wall in the corner store. "Hey, Chin, tear off one of those phone number strips for me."Hyphenated adjectives or nouns from a verb phrase demonstrate language evolution in action. It's easy to see where tearing off a strip led to tear-off as a descriptor for the strip. The next phase in evolution is often the losing of the hyphen.
He showed it to her and she held up an evidence photo on her iPhone. "Our victim had one of these same tear-off strips in his pocket. We need to check it out."
Danny held up a finger as he fished out his phone. "Hang on a sec. I gotta check up on Gracie. She had a dental checkup today."The word checkup has been around so long, we don't use the hyphen anymore, similar to the way we don't use 'phone with an apostrophe to indicate the abbreviation of telephone.
The hyphens don't always get lost. Cops have been investigating break-ins since long before even the original Hawaii Five-0. But sometimes language evolves pretty quickly, such as accepting that Chin checks his account logins rather than his log-ins.
So how do you know when to hyphenate and when to use a single word? Your guide will usually be the dictionary. It will list most adjectives and nouns formed from verb phrases either with a hyphen or as one word.
Sometimes it takes dictionaries a little while to catch up to usage, though. If you can't find your word in the dictionary, it will likely be newly created, so hyphenated. If you've found common usage of the new word in books, news stories, or blogs, follow what seems to be the most accepted spelling.
The most important thing is to not use your adjectives or nouns in place of verbs! The verb phrase will always be separate words.
Here are a few more verb phrases with their adjective and/or noun counterpart:
get away – getaway
check out – checkout
cut off – cut-off
round up – roundup
drop off – drop-off
set up – setup
break down – breakdown
back up – backup
lay out – layout
pay off – payoff
touch up – touchup
run away – runaway
You can read an amusing argument that "Login is not a verb" at notaverb.com, where there are even more examples.
How many more nouns or adjectives coming from verb phrases can you think of?