The picnic for boys are situated by the hill some two hours away from the city.

Correct sentence: The picnic for boys is situated by the hill some two hours away from the city.

The matching of singular verbs with singular subjects and plural verbs with plural subjects is the act of ensuring subject-verb agreement. The verb "are" is plural, and the subject "picnic" is singular. Therefore, the verb "is" agrees with the subject "picnic".

In addition to success in the film industry, many actors is also successful in business.

Correct sentence: In addition to success in the film industry, many actors are also successful in business.

The noun "actors" is already plural, and the verb "is" is plural. So the verb "are" agrees with the subject "actors".

Everybody in the office were afraid of the manager, so they tried to hide from him.

Correct sentence: Everybody in the office was afraid of the manager, so they tried to hide from him.

Certain indefinite pronouns such as "everybody" may seem like they should be plural (since it does talk about more than one person), but they are actually singular. Therefore, only the singular verb "was" is appropriate here.