Spanish indefinite pronouns are unspecific and are used in place of nouns. They can be the subject of a sentence, the object of a verb, or the object of a preposition. For example:

• Todos están aquí.
  Everyone is here.

• Compró algo.
  He bought something.

• Tengo un regalo para alguien.
  I have a gift for someone.

The Spanish indefinite pronouns are:

algo                - something 1 
alguien             - someone 1
alguno, alguna      - a certain one 1, 2
algunos, algunas    - some, a few 2, 3
cada uno            - each one 1, 2, 3 
cualquiera          - anyone, whoever 1 
mucho, mucha        - a lot, much, many 3 
muchos, muchas      - a lot, much, many 3 
otro, otra          - another one 1, 2 
otros, otras        - others 2 
quienquiera         - whoever 1
todo                - everything 1 
todos               - everybody 3 
uno                 - one 1, 3 
varios, varias      - some, several 2, 3 

Notes:

1 These pronouns take the third person singular form of the verb.

• ¿Todo está bien?
  Is everything ok?

• Alguien debe saber.
  Someone must know.

2 These pronouns must always have an antecedent (something they refer to in the same or a preceding sentence).

• Perdí mi pluma, entonces tengo que comprar otra.
  I lost my pen, so I have to buy another one.

• Tiene 50 libros y quiero leer cada uno.
  He has 50 books and I want to read each one.

3 These pronouns can be modified with de + ellos, ellas, nosotros, vosotros, Uds., or a noun.

• Uno de vosotros tiene que ayudarme.
  One of you has to help me.

• Algunos de los estudiantes están aquí.
  Some of the students are here.