Quick answer
Maybe as one word is an adverb meaning perhaps — maybe we should leave. May be as two words is a verb phrase, "may" plus "be" — it may be too late. The test: if you can substitute "perhaps", use one word.
The core difference
An adverb and a verb phrase.
- maybe (adverb) — perhaps: maybe she forgot.
- may be (verb phrase) — might be: the answer may be simpler.
The test
Substitute perhaps. "Perhaps she forgot" works → one word. "The answer perhaps simpler" does not work → two words, because the sentence needs a verb and be is supplying it.
A second check
If the sentence already has another main verb, you probably want maybe. If be is doing the verb's job, you need the two-word form.
Frequently asked questions
Which means perhaps?
Maybe, as one word.
What is the test?
Substitute "perhaps". If the sentence still works, use one word.
How do I spot the two-word form?
If "be" is doing the work of the verb, you need may be as two words.