Yes. The plural is fossils.
Yes, the word 'fossil' is a singular, common, concrete noun; a word for the remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock; a word for a thing.
Yes. Hay is a singular noun. A Singular noun means one item only. So technically, hay is a singular noun.
The noun 'theory' is a singular noun. The plural noun is 'theories'.
The noun 'mice' is the plural form of the singular noun 'mouse'.
Yes the word question is a singular noun. The plural noun is questions.
The noun mailbox is the singular form; the plural is mailboxes.
The noun school is the singular form.The plural noun is schools.
The noun 'fun' is a singular non-count noun, it has no plural form.
Feet is a plural noun. Foot is a singular noun.
The noun juggler is a singular noun. The plural noun is jugglers.
The singular noun is "louse" and the plural noun is "lice."
The noun family is the singular form; the plural form is families.