Mistress is already singular.
There is no apostrophe at the end of the word mistress. An apostrophe goes at the end of PLURAL possessives ending in -s, not singular ones. The possessive singular of mistress is mistress's, and the plural possessive is mistresses'.
The singular form of the word flagella is flagellum.
Guma is an Italian slang term for the English word "mistress".Specifically, the word is a feminine noun in its singular form. It also may be found spelled gooma or goomah. It may be pronounced "GOO-ma" or "goo-MA".
Criterion is the singular form of criteria.
The word 'duo' is the singular form; the plural form is duos.
The word mistress is a noun. The plural form is mistresses.
Magistram is the accusative singular form of the word magistra, which means "mistress." The accusative case is used for direct objects and for the objects of certain prepositions such as ad ("to") and apud ("near; at the home of").
The singular form of the word "señores" is "señor."
That is the singular form; the plural form is those.
The singular form is commando.
'Jackhammer' is the singular form.
The singular possessive form of jury is jury's