From the many rules for subject/verb agreement, we are picking the most difficult one to discuss.
First, the groundwork: When the subject is singular, the verb is singular; when the subject is plural, the verb is plural.
...One of the boys HAS to be the leader.
...Several of the boys HAVE to be the leaders.
...Each of the employees IS being questioned.
...All of the employees ARE being questioned.
When the verb you are analyzing is in a dependent clause, you have to look for the word the clause modifies to determine the form of the verb. If it modifies a singular word, the verb is singular; if it modifies a plural word, the verb is plural.
...The man who WAS driving that car worked for me.
...The men who WERE driving those cars worked for me.
...A building that HAS several floors suits us better.
...Buildings that HAVE several floors suit us beeter.
Part 2 tomorrow!!
Happy punctuating!
Margie