Answer: * When someone is tired, he can't think very well. OR
When someone is tired, she can't think very well.
Principals can be more successful when they treat students with respect.
Explanation: Traditional rule: if the antecedent (usually a noun)is singular, use a singular pronoun: he, she, it.
If the antecedent is plural, use a plural pronoun: they, them
* This is not a good example because the subject/antecedent is an indefinite pronoun. There is a recent trend to use 'they' as the pronoun for unidentified gender rather than the traditional 'he.' But it is good to know that in older texts 'he' was used to refer to both masculine and feminine antecedents.