Things can get complicated when we attempt to determine the number of indefinite pronouns like someone, somebody, anyone, anybody, everyone and everybody. For an extensive discussion of these problems, see:

Pronouns and Pronoun Antecedent Agreement (Darling et al)

Pronouns (definitions) (Darling)
Pronouns & Nouns Combined (Darling et al)
Pronoun Cases (Darling et al)
Pronoun Consistency (Darling et al)

Using Pronouns Clearly (Purdue OWL)
Pronoun Case (Purdue OWL)
Gender Problems (Darling et al)

Avoid using indefinite pronouns like this, that, these and those. Use these words as adjectives instead. 
Example:

Using Pronouns Clearly (Purdue OWL)

Pronouns and Pronoun Antecedent Agreement (Darling et al)
Pronouns (definitions) (Darling)
Pronouns & Nouns Combined (Darling et al)
Pronoun Cases (Darling et al)
Pronoun Consistency (Darling et al)
Pronoun Case (Purdue OWL)
Gender Problems (Darling et al)
Consistency: Verbs and Pronouns (Darling et al)

Pronoun Usage  (Darling et al)
Pronoun Usage II  (Darling et al)
Pronoun Usage III (Darling et al)
Pronoun Cases I (Darling et al)
Pronoun Cases II (Darling et al)
Who (cgi) Who II (Darling et al)
Which, That and Who  (Darling et al)
Practice: Pronouns (Darling et al)
Practice: Pronouns II  (Darling et al)
Practice: Pronouns III (Darling et al)

Choosing the right pronoun form to use depends on whether the pronoun is functioning as the subject or object in the sentence.

Pronoun Cases (Darling et al)
Pronoun Case (Purdue OWL)

For extensive discussions of the use of who and whom, see
Who and Whom (Darling et al)

Pronouns (definitions) (Darling)

Pronouns and Pronoun Antecedent Agreement (Darling et al)
Pronouns & Nouns Combined (Darling et al)
Pronoun Consistency (Darling et al)
Using Pronouns Clearly (Purdue OWL)
 Gender Problems (Darling et al)
Consistency: Verbs and Pronouns (Darling et al)