Learn Finnish: Olin piirtänyt kolmion, ennen kuin opettaja saapui. - I had drawn a triangle before the teacher arrived.

Olin piirtänyt is the pluperfect (past perfect) tense, meaning "I had drawn". We use it to describe an action that was already fully completed before another past action happened. Here, the drawing was finished before the teacher arrived (saapui). It is formed using the past tense of olla (olin) plus the past active participle of the main verb (piirtänyt).

Why is the word for "triangle" in the genitive form (kolmion)?

It is acting as a total object. In Finnish, if an action is completed and affects the whole object, the object takes the genitive case (often called the genitive-accusative). Because you finished drawing the entire triangle, you use kolmion. If you had been interrupted in the middle of drawing it, you would use the partitive case: olin piirtämässä kolmiota.

Why do we need the word kuin after ennen here?

When "before" connects two full clauses—meaning both sides have a conjugated verb, like olin piirtänyt and saapui—you must use the pair ennen kuin. You only use ennen by itself when it is followed directly by a noun, such as ennen tuntia (before the lesson).

Could I use tuli instead of saapui for "arrived"?