sub vs fill in

sub is a noun but fill in is not a noun.

Word	Noun	Adjective	Verb	Adverb
sub	Yes	No	Yes	No
fill in	No	No	Yes	No

As verbs, sub and fill in are synonyms defined as:

Other synonyms of sub include stand in, substitute.

sub (noun)	fill in (noun)
a submersible warship usually armed with torpedoes
a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States

sub (verb)	fill in (verb)
be a substitute	supply with information on a specific topic
	write all the required information onto a form
	represent the effect of shade or shadow on
	be a substitute