astern vs aft

astern is not an adjective while aft is an adjective.

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

As adverbs, astern and aft are synonyms defined as:

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

Other synonyms of astern include abaft.

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

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