aft vs astern

aft is an adjective but astern is not an adjective.

Word	Noun	Adjective	Verb	Adverb
aft	No	Yes	No	Yes
astern	No	No	No	Yes

As adverbs, aft and astern are synonyms defined as:

aft and astern: at or near or toward the stern of a ship or tail of an airplane

Other synonyms of aft include abaft.

aft (adjective)	astern (adjective)
(nautical, aeronautical) situated at or toward the stern or tail

aft (adverb)	astern (adverb)
at or near or toward the stern of a ship or tail of an airplane	(of a ship or an airplane) behind
	at or near or toward the stern of a ship or tail of an airplane
	stern foremost or backward