i have prepared my grammar notes from various sources- my high school grammar book, Wren and Martin, Spidey's notes, Sahil's notes. So i do not really remember from where i picked up this grammar rule, but i will tell u exactly what i have written, so that you may include it in your compilation.
Indefinite pronouns:
eg: One hardly knows what to do
somebody has stolen my watch
Did you ask anybody to come?
All these pronouns refer to people/ things in a general way, but do not refer to any person/ thing in particular. So they are called indefinite pronouns.
We use a singular verb after indefinite pronouns.
After words with everybody, somebody, anybody, someone etc we use plural words even though the verb is singular. We ca use he, she, him/her etc with someone/someone when we know the person's gender.
eg: I shall be glad to help everyone of my boys in his/their studies.
"their" is usually accepted correct for indefinite pronouns.
Hope this helps!