Learn Finnish: Kaupassa tuli ostettua ylimääräinen paita. - I accidentally bought an extra shirt at the store.

The inadvertent action construction often drops the person entirely if it is clear from the context. If you wanted to explicitly state who did it, you would use the adessive case: minulla tuli ostettua. Without it, it translates to a general "I ended up buying" or "Someone ended up buying" depending on the context of the conversation.

Why is the object ylimääräinen paita in the nominative case rather than the accusative/genitive ending in -n?

In this construction, the verb tuli acts as an impersonal verb without a standard grammatical subject. Just like with necessive verbs (such as minun täytyy ostaa) or the standard passive voice, the total object takes the basic nominative case when there is no active subject to trigger the -n ending.

How is the form ostettua built from the basic verb ostaa?

It uses the passive past participle (ostettu), which roughly means "bought", and puts it into the partitive case by adding -a. The formula for this unplanned action structure is always the 3rd person singular tuli followed by this specific partitive passive participle form.

Does this structure mean I bought the shirt completely by mistake, like grabbing the wrong item?

Not necessarily a literal mistake. While it is often translated as "accidentally," this structure is very commonly used to mean "I ended up buying" or "I happened to buy." It softens the action, making it sound spontaneous, unplanned, or slightly impulsive rather than a strict error.