Indefinite pronouns are pronouns used for unspecified people and things. You can use them when you don't know who or what is undertaking the action - "man" (one), "jemand" (someone), "etwas" (something), "jede(r/s)" (each), "nichts" (nothing), "niemand" (no-one) - or when you can't or don't want to say exactly how many are involved - "alle" (everyone), "alles" (everything), "einige" (some), "viele" (many).

Examples:
Jemand muss den Maibaum aufstellen.
Es ist alles fertig für die Feier!

Indefinite pronouns can refer to people, other living beings, or objects. Some can only be used for people. Others can only be used for anything besides people.

Indefinite pronouns that refer to people
"niemand" (no-one) and "man" (one - which is comparable to the generic "you") are examples of indefinite pronouns that refer only to people. They are used alone and take the place of a noun. They can only be used in the singular.

Examples:
Niemand isst Würstchen und Kartoffelsalat.
Auf der Landstraße darf man Fahrrad fahren.

Indefinite pronouns that refer to things, but not people
"etwas" (something), "nichts" (nothing) and "alles" (everything) are examples of indefinite pronouns that refer to things and concepts, but not people. They are also singular pronouns. "etwas" and "nichts" can only be used in the singular. The plural of "alles" is "alle" and can refer to people.

Examples:
Es ist nichts passiert.
Ich kann doch nicht alles alleine machen.

Indefinite pronouns that refer to people, other living beings, concepts or things
"jede/r/s" (everyone in the female, male and neuter forms respectively) and "alle" are examples of indefinite pronouns that can refer to people, living beings or things. "jede/r/s" is a singular pronoun. "alle" is a plural pronoun.

Examples:
Das kann jeder sagen.
Alle feiern, auch der Präsident.