Someone, everyone...

[Latoof]

How should we deal with words like
someone, everyone, anyone... Are they treated like a third person singular; I mean adding -s?

[Tdol]

Yes:
Someone has taken mybook.
Everone was there
etc.

[Abdo]

We use singular verb form after them as explained by tdol:
Someone has taken mybook. / Everone was there
But if we refer to these pronouns later we use plural forms
e.g. Someone has taken my book and forgot theirs.
Someone has taken my book, haven't they?

[Tdol]

Some still use the third person singular, though it sounds more and more old-fashioned to me.