As their name implies, indefinite pronouns are pronouns used to refer to non-specific people and things.

Some indefinite pronouns are only used to refer to people. These indefinite pronouns are somewhat easy to memorize as most of them end with -one, –body, and -ever.

anybody	no one	whosoever
anyone	somebody	whomever
everybody	someone	whomsoever
everyone	they	you
nobody	whoever

Notice that you and they are typically used as personal pronouns, but they can also be used generically as indefinite pronouns. In such cases, you is sometimes called generic you, impersonal you, or indefinite you. The pronoun you and its other second-person counterparts are often used in a generic sense in informal writing. They is sometimes used generically to refer to people in general or to an unspecified group of people.

You should never underestimate your opponent.
They said our dog should make a full recovery.

Certain other indefinite pronouns are only used to refer to things. Several of them conveniently end with thing, but the rest may simply need to be memorized.

anything	little	such
enough	much	this
everything	nothing	whatever
less	something	whichever

Many indefinite pronouns can be used to refer to things or to people, depending on the context.

another	fewer	one
any	many	other
both	more	others
each	most	plenty
either	neither	several
few	none	some

It's nice to share your experience with another. (person)
These cookies are great. I'll have another. (thing)
John and Jack are avid outdoorsman. Either would make a great guide. (person)
I don't care which color. Either is fine. (thing)
For many are called, but few are chosen. — Matthew 22:14 (person)
Millions of books have been written, but few have become bestsellers. (thing)

Singular or Plural?

Take note that many indefinite pronouns can be singular or plural depending on how they are being used. Take a carful look at the context and verb agreement to determine if an indefinite pronoun is singular or plural.

All is well. (singular)
All are welcome. (plural)
Such is life. (singular)
And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God. — I Corinthians 6:11

Some pronouns are obviously singular, such as those ending with one. Others are clearly plural such as both and others. However, many pronouns that may appear to be plural may actually refer to a group as a singular unit.

Everybody is excited for the weekend.

Although everybody is clearly used to refer to a lot of people, it is referring to all those people as one unit. Notice that we use the third person singular verb is rather than the third person plural verb are to agree with everybody.