Learn Hardcore Finnish: Pomo hyväksyi ehdotuksen, koska se oli selkeä. - The boss accepted the suggestion because it was clear.

Ehdotuksen carries the accusative/genitive -n ending, marking it as the object of a completed action. In Finnish, when a transitive verb denotes a finished, whole action (here hyväksyi “approved”), its object takes the -n form. If the action were partial or ongoing, you’d use the partitive (e.g. hyväksyi ehdotusta).

When should I use the partitive case for objects instead of the accusative/genitive -n?

Use the partitive case (e.g. ehdotusta) when the action or quantity is:
• Incomplete or in progress
• Referring to an indefinite amount
• Expressing feelings, needs, or likes/dislikes
For example, luin kirjaa (“I was reading the book”) uses the partitive. For a completed, countable action, switch to -n.

What does the pronoun se refer to, and why not hän?

Se is the inanimate third-person pronoun in Finnish. It refers back to ehdotus (a non-human object). Hän is used only for people (and sometimes animals). Since a “proposal” isn’t animate, you must use se.

Why is the adjective selkeä not inflected with a case ending?

After the verb olla (“to be”), adjectives are predicative complements and stay in the nominative form, agreeing in number (here singular) with their subject. Since se is nominative singular, selkeä remains plain. If you instead used the adjective attributively before a noun, you’d still have nominative selkeä ehdotus, but with other cases it could change (e.g. genitive selkeän ehdotuksen).

How do I know that se (not pomo) is the subject of oli selkeä?

Finnish subordinate clauses normally need an explicit subject. In koska se oli selkeä, se is the subject and oli the verb. Pomo is the subject of the main clause (Pomo hyväksyi ehdotuksen) and cannot double as the subject of the subordinate clause.

Why is the conjunction koska used here, and how does it differ from sillä or kun?

Koska introduces a subordinate causal clause (“because it was clear”).
Sillä is a coordinating conjunction meaning “for/because,” used mid-sentence:
Pomo hyväksyi ehdotuksen, sillä se oli selkeä.
Kun primarily means “when,” but colloquially can mean “because.” To avoid ambiguity, use koska for clear causal meaning.

Why is there a comma before koska?

In Finnish, a comma separates main and subordinate clauses. When the koska-clause follows the main clause, you place a comma before it. The same applies if the koska-clause comes first—you still use a comma after it.

How is the past tense formed in hyväksyi?

Hyväksyi is the third-person singular past of hyväksyä (“to approve”). Finnish past tense for many verbs is formed by adding -i- plus personal endings to the verb stem. Conjugation for hyväksyä looks like:
minä hyväksyin – sinä hyväksyit – hän/se hyväksyi – me hyväksyimme – te hyväksyitte – he hyväksyivät