No, the noun 'toucan' is a countable noun the plural noun is toucans.
It is both a singular or plural noun.
Yes, the noun sheep is both singular and plural.
The word "ground" can be both singular and plural. Singular: "The ground is wet." Plural: "The grounds of the park are well-maintained."
Fish is both singular and plural.
Epidermis is both singular and plural
"Lens" is both singular and plural.
Fish is both singular and plural.
The verb reviewed is used after both singular and plural nouns.
The plural of toucan is toucans.
Verbs are not plural or singular. Melting comes after both plural and singular nouns.
"You" can be both singular and plural.
Mumps is plural. But it is both singular and plural is construction.