15. nobody -- no body

There was nobody (no person) at the information desk.
No body (no group) of employees is more helpful than yours.

16. onto -- on to

He lost control of the car and drove onto the sidewalk. ("Onto" implied movement toward and then over).
Let's go on to the next problem. (On is an adverb in the verb phrase "go on"; "to" is a preposition).
She then went on to tell about the accident. ("On" is part of the verb phrase went on; to is part of the infinitive to tell.)

17. someday -- some day

Please set up a meeting with Marcia someday (on an unspecified day) this week.
Please set up a meeting with Marcia for some day next week. (Two words when the object of a preposition as "for").