Learn Finnish: Hän uskoi ystäviinsä, joten hän ei pelännyt tulla petetyksi. - He believed in his friends, so he wasn't afraid of being betrayed.

This is the translative participle construction, which expresses experiencing a passive action. It combines the verb tulla (to become) with the passive past participle of the verb pettää (to betray). The participle takes the translative case ending -ksi, so the phrase literally means "to become betrayed".

Why is ystäviinsä in the illative case here instead of the partitive?

The verb uskoa requires the illative case (into) when it means "to believe in" someone or something, such as a friend or an idea. If you only want to say you believe what someone is saying, you would use the partitive case, as in uskon sinua (I believe you).

What are the different suffixes attached to the word ystäviinsä?

It is built on the root ystävä (friend). First, we add the plural marker -i- and the illative ending -in, forming ystäviin (into the friends). Then, the third-person possessive suffix -nsä (his/her) is added. When a possessive suffix is attached to an illative form ending in -n, that final -n drops out, resulting in ystäviinsä.

Why is the word pelännyt used here instead of the standard past tense form pelkäsi?