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 "that" is "it."
it is already in its singular form.
The singular form is cuff.
"Stratum" is the singular form of stratum. The plural form is "strata".
The form hypothesis is the singular form. The plural form is hypotheses.
A singular form of dice is a die
The singular form of feet is foot.
The singular form of Oases is, Oasis.
The singular form of chromosomes is a chromatid.