Answer #1 'Monsieur' is already singular. Monsieurs with the "s" is plural. Answer #2 The noun 'Monsieur' combines the possessive adjective 'mon' ['my] and the noun 'sieur' ['lord'] to mean 'mister' or 'sir' nowadays. It's a masculine gender noun that's in the singular. The plural form is Messieurs, as in 'Messieurs et Mesdames' [in reversed order as 'Ladies and Gentlemen' in English].
singular: monsieur plural: messieurs
M. monsieur (singular) is abbreviated: m. (no 'r') messieurs (plural) is abbreviated: mm.
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "cuffs" is cuff.
The singular form of feet is foot.
The singular form of "dice" is "die."
The singular form of "hypothysis" is "hypothesis."
The singular form is ombudsman.
The singular form is hilum.
The singular form is money.
it is already in its singular form.
The singular form in stimulus.