Welcome to the Grammar Minute, where we’re saving the English language sixty seconds at a time! I’m Lauren Smyth, and how do you know when a word should  be two words?
Take a word like “anytime.” It could be “anytime,” or “any time” with a space. See also “anyone” or “altogether” or any number of other words that are compounds of two words.
It depends on what part of speech you’re using. In a sentence like “I could go anytime,” “anytime” is one word because it’s describing the verb—an adverb. If you changed the sentence to be “I could go at any time,” then “any time” is an adjective and noun, and therefore two separate words.
 Unfortunately, there’s not a blanket rule that every descriptor is one word and every noun is two words. For example, “I could be anyone” is a noun and one word, but “I could handle any one of those options” is also a noun and two words. 
This is one of those cases where it just comes down to being very familiar with parts of speech and general usage. You can get there by listening to our episode on the parts of speech and by reading extensively.
That’s your Grammar Minute! Visit thegrammarminute.com for more tips and tricks.