He did it to show the difference in their status and education.
Iambic pentameter is an elevated style, and Shakespeare created noble and educated characters who spoke in it. On the other hand, there were commoners who obviously did not belong to the same class as the nobles and did not receive the same education (if they had any education to begin with) so he wanted to show those differences through their speech. Nobles spoke in iambic pentameters, and commoners in blank verse or prose.
The bolded pronoun in the given sentence is the word "which". This pronoun is an example of an interrogative pronoun. Interrogative pronouns are pronouns that are used to introduce a question. The antecedents of interrogative pronouns are unknown because it is what is being asked in the sentence that is missing.
Trepak is a subdivision of classical dance from Tchaikovsky's "The Nutcracker" and its origin takes sources from classical Russian and Ukranian folk dances. Trepak is on of the most popular dances from the Tchaikovsky's piece, and its cheerful and powerful rhythm requires enormous strength and resistance from dancers.
