commonplace vs prosaic

commonplace is a noun but prosaic is not a noun.

Word	Noun	Adjective	Verb	Adverb
commonplace	Yes	Yes	No	No
prosaic	No	Yes	No	No

As adjectives, commonplace and prosaic are synonyms defined as:

Other synonyms of commonplace include humdrum, unglamorous, unglamourous.

commonplace (noun)	prosaic (noun)
a trite or obvious remark

commonplace (adjective)	prosaic (adjective)
not challenging; dull and lacking excitement	not challenging; dull and lacking excitement
repeated too often; overfamiliar through overuse	lacking wit or imagination
completely ordinary and unremarkable	not fanciful or imaginative