Learn Turkish: Ayı kükrediği için ormanda sürünmek zorundaydı. - Because the bear roared, he had to crawl in the forest.

To say 'because [subject] did [action]', Turkish turns the verb into a noun-like clause. It adds the participle suffix -dIk plus a possessive suffix to the verb stem, creating kükrediği (literally 'its roaring'). You cannot place a fully conjugated verb like kükredi (it roared) directly before için.

Should 'bear' have a genitive suffix here, like ayının kükrediği için?

Technically, yes. In a -dIğI için clause, the subject of the action usually takes the genitive case (ayının = the bear's). However, native speakers often drop the genitive on the subject in these clauses in everyday speech, leaving just ayı, especially when it isn't a specific personal pronoun.

Literally, how does zorundaydı mean 'he had to'?

It comes from the noun zorun (obligation/compulsion) + -da (the locative suffix 'in/at') + -(y)dı (the past tense copula 'was'). Literally, it translates to 'he was in the obligation'. In Turkish, you pair the infinitive form of the verb (sürünmek) directly with zorunda to express that someone has to do something.

The current lesson is about negative counterfactual past ability. How would we change this sentence to mean 'he wouldn't have been able to crawl'?