As nouns the difference between fern and a

is that fern is any of a group of some twenty thousand species of vascular plants classified in the division Pteridophyta that lack seeds and reproduce by shedding spores to initiate an alternation of generations while a is The name of the Latin script letter A/a.

As a proper noun Fern

is {{given name|female}} from the fern plant.

As a letter A is

: Apple starts with A.

As a number A is

: The item A is "foods", the item B is "drinks".

As a symbol A is

the highest rank on any of various scales that assign letters.

As an article a is

one; any indefinite example of; used to denote a singular item of a group.

As a preposition a is

To do with position or direction; In, on, at, by, towards, onto.

As a verb a is

(have) Have.

As a pronoun a is

he.

As an interjection a is

a meaningless syllable; ah.