Learn Finnish: Isä on autotallissa korjaamassa autoa. - Father is in the garage fixing the car.

This is the third infinitive inessive form, which is the focus of this lesson. When combined with the verb olla (here as on), it describes an action that is currently ongoing. It translates perfectly to the English continuous form, 'is fixing'.

How do you form korjaamassa from the dictionary verb korjata?

korjata is a verb type 4. To make this form, you first find the strong verb stem, which is korjaa-. Then, you add the ending -massa (or -mässä, depending on vowel harmony). Since korjaa- has back vowels (o, a), it takes -massa.

Both autotallissa and korjaamassa end in -ssa. Is that a coincidence?

Not at all! The ending -ssa is the inessive case, meaning 'in'. autotallissa literally means 'in the garage'. The third infinitive -massa form is actually built using this exact same ending. In a literal sense, the sentence is saying father is in the garage 'in the act of' fixing the car.

Why is the car in the partitive case (autoa) instead of the basic form (auto)?

Because the action is ongoing and incomplete. The -massa form strongly emphasizes that he is currently in the middle of fixing the car. When an action is incomplete, the object receiving the action must take the partitive case.