Learn Finnish: Sekä elokuvan loppu että sen toteutus olivat hyvin traagisia. - Both the movie's ending and its execution were very tragic.

This is a fixed correlative conjunction meaning "both ... and". Here it links the two parts of the subject: the ending (elokuvan loppu) and the execution (sen toteutus).

Since loppu (ending) and toteutus (execution) are both singular words, why is the verb olivat in the plural?

When you connect two singular subjects using sekä ... että, they combine to form a plural subject. Because the two things act together, the verb must take the plural form olivat instead of the singular oli.

What does the pronoun sen refer back to here?

The word sen is the genitive form of se (it). In this context, it refers back to elokuva (movie). Using sen toteutus (its execution) safely avoids repeating the word for movie, just as it does in English.

Why is the adjective traagisia in the partitive plural form instead of nominative plural?

In Finnish, when a sentence with the verb to be has a plural subject, the adjective describing it (the predicative adjective) almost always takes the partitive plural. Since the combined subject "both the ending and the execution" is plural, the adjective takes the partitive plural form traagisia.