Learn Finnish: Virstanpylväs, joksi tilanne muuttui, oli iso käännekohta. - The milestone the situation turned into was a big turning point.

Joksi is the translative case of joka. The verb muuttua (to change, to turn into) requires the translative case for the result of the change. Because the situation turned into the milestone, the relative pronoun referring back to it gets the translative -ksi ending.

English uses 'into' here. Could I use the illative case johon instead of joksi?

No, because Finnish separates physical movement from a change of state. You use the illative (johon) when moving into a physical space. When something changes its form, role, or state, you must use the translative case (joksi). The situation isn't physically entering a milestone; it's becoming one.

The word virstanpylväs looks very long. What is it made of?

It is a compound word made of virstan (the genitive form of virsta, an old unit of distance similar to a mile) and pylväs (pillar or post). A 'mile-post' translates perfectly to 'milestone' in a metaphorical sense.

Why is iso käännekohta in the nominative case (basic form) rather than the partitive?

In the main clause, iso käännekohta describes what the milestone was. This makes it a subject complement linked by the verb olla (to be). Because the main subject (virstanpylväs) is a singular countable noun, its complement also stays in the basic nominative form.