only vs simply

only is an adjective but simply is not an adjective.

Word	Noun	Adjective	Verb	Adverb
only	No	Yes	No	Yes
simply	No	No	No	Yes

As adverbs, only and simply are synonyms defined as:

Other synonyms of only include but, just, merely.

only (adjective)	simply (adjective)
exclusive of anyone or anything else
being the only one; single and isolated from others

only (adverb)	simply (adverb)
with nevertheless the final result	and nothing more
without any others being included or involved	in a simple manner; without extravagance or embellishment
and nothing more	absolutely; altogether; really
never except when	(used for emphasis) absolutely
in the final outcome
as recently as
except that