separate vs single out

separate is a noun but single out is not a noun.

Word	Noun	Adjective	Verb	Adverb
separate	Yes	Yes	Yes	No
single out	No	No	Yes	No

As verbs, single out is a hyponym of separate; that is, single out is a word with a more specific, narrower meaning than separate:

As verbs, separate and single out are synonyms defined as:

Other synonyms of separate include discriminate.

separate (noun)	single out (noun)
a garment that can be purchased separately and worn in combinations with other garments
a separately printed article that originally appeared in a larger publication

separate (adjective)	single out (adjective)
independent; not united or joint
standing apart; not attached to or supported by anything
separated according to race, sex, class, or religion
have the connection undone; having become separate

separate (verb)	single out (verb)
become separated into pieces or fragments	select from a group
divide into two or more branches so as to form a fork	treat differently on the basis of sex or race
arrange or order by classes or categories
mark as different
move or break apart
force, take, or pull apart
divide into components or constituents
go one's own way; move apart
make a division or separation
treat differently on the basis of sex or race
separate into parts or portions
discontinue an association or relation; go different ways
act as a barrier between; stand between