A single invitation is different from plural invitations.
Yes, the noun 'invitations' is a common noun, a word for any invitations of any kind.
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 word invitation is a noun. The plural noun is invitations.
The noun 'fun' is a singular non-count noun, it has no plural form.
An example of a singular noun for a school could be "institution."
The noun juggler is a singular noun. The plural noun is jugglers.
Feet is a plural noun. Foot is a singular noun.
The noun family is the singular form; the plural form is families.