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.
Criterion is the singular form of criteria.
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".
The word 'duo' is the singular form; the plural form is duos.
The word mistress is a noun. The plural form is mistresses.
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 word dog is the singular form.
The singular possessive form of jury is jury's