The noun 'fun' is a singular non-count noun, it has no plural form.
The noun 'laugh' is a singular, common, concrete noun; a word for the act of laughing, laughter, fun, amusement; a word for a thing.
The word circus is a singular noun. You would use the third-person singular pronoun: it.
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.
Feet is a plural noun. Foot is a singular noun.
The noun juggler is a singular noun. The plural noun is jugglers.
The noun school is the singular form.The plural noun is schools.
The singular noun is "louse" and the plural noun is "lice."
The noun family is the singular form; the plural form is families.