The adjective "old-fashioned" does not take singular or plural because it is an adjective. Only nouns (or other words acting as nouns) can be singular or plural.
For example: Old-fashioned lady is one lady who's old-fashioned; old-fashioned ladies can be 2 or more ladies who are old-fashioned. In both cases, the adjective is the same; only the noun changes according to number.
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "hypothysis" is "hypothesis."
The singular form of Oases is, Oasis.
A singular form of dice is a die
"Wash" is the singular form of "wash."
The singular form of the demonstrative pronoun 'these' is this.
The singular form of feet is foot.
The singular form of "hypothysis" is "hypothesis."
The singular form is ombudsman.
The singular form is hilum.
The singular form is cuff.
The singular form is money.
it is already in its singular form.
The singular form in stimulus.
The singular form for the noun families is family. The singular possessive form is family's.
The singular form for the plural noun stories is story.
The form plumber is the singular form; the plural form is plumbers.