Learn Finnish: Minua harmittaa, koska komedia loppui. - It vexes me, because the comedy ended.

The suffix -ttaa often creates a verb meaning "to cause a feeling". Here, it is added to the noun harmi (annoyance, pity) to create harmittaa, meaning "to cause annoyance" or "to vex". Instead of saying "I am annoyed", Finnish expresses this as "It annoys me".

Why is the pronoun minä (I) in the partitive form minua?

With emotion verbs ending in -ttaa or -ttää, the person feeling the emotion is structurally the object of the sentence, receiving the feeling. Because emotions are abstract and ongoing, this object must take the partitive case.

Why doesn't the verb harmittaa have a subject like "se" (it)?

In Finnish, emotion verbs like harmittaa are frequently used in the 3rd person singular without a formal subject pronoun like se. The true source of the annoyance is simply explained by the following clause (koska komedia loppui), so an explicit "it" is unnecessary.

Why do we use loppui instead of lopetti for "ended"?

loppui comes from the intransitive verb loppua (to end / to run out), which is used when something ends on its own. If a person actively ended or finished something, you would use lopettaa, which is actually another causative verb! Since the comedy ended by itself, loppua is correct here.