The noun 'series' is the plural form of an uncountable (mass) noun.
singular: A series of playoffs took place before the tournament.
plural: He has published three series of adventure novels, each with a different hero.
Series is a noun that has the same form in the singular and the plural. There is no need for an apostrophe, unless it is in the possessive case: series's for the singular possessive and series' for the plural possessive.
The singular form is it, the plural form is they(subjective) and them (objective).
The word crisis is singular. The plural form of crisis is crises.
The singular possessive form is lesson's.
The singular possessive form is grotto's.
Series is a noun that has the same form in the singular and the plural. There is no need for an apostrophe, unless it is in the possessive case: series's for the singular possessive and series' for the plural possessive.
The singular form of the word flagella is flagellum.
Criterion is the singular form of criteria.
The word 'duo' is the singular form; the plural form is duos.
The singular form of "series" is "series." It remains the same whether referring to one or multiple sets of related things.
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
The word "biopsies" is the plural form of the word. The singular form is "biopsy."