18. someone -- some one

Some one must sign for the package.
Some one of us must sign for the package.
Someone likes me.
Note: Both imply any person.
Use two words if followed by an of phrase or used to mean "one of a number of people."

19. upon -- up on

His statements were based upon experimental data. (Upon is a preposition.)
Please follow up on the Meeks case. (Up is part of the verb phrase follow up; on is a preposition).

20. whoever -- who ever

Whoever (anyone who) said that should be fired.
Who ever made such a statement? ("Ever" is an adverb).