fill in vs sub

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

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

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

Other synonyms of fill in include stand in, substitute.

fill in (noun)	sub (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

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