Learn Finnish: Järvelle muodostuu jäätä yöllä.

Muodostua (to form, to take shape) is an automative verb derived from the transitive verb muodostaa (to form something). The -tua suffix shows that the action happens by itself, without a specific external agent doing it. Here, the ice forms on its own.

Why is the subject jäätä in the partitive case instead of the nominative jää?

This is an existential sentence expressing that a new substance appears. Because jää (ice) is an uncountable mass noun, it takes the partitive case (jäätä) to show that an unspecified amount of ice is forming.

Why does the sentence use the allative case järvelle (onto the lake) rather than the adessive järvellä (on the lake)?

In Finnish, verbs of appearing, forming, or building often require a directional case rather than a static location case. Since the ice is appearing onto the surface of the lake, the directional case järvelle is used.

Why is the word order Järvelle muodostuu jäätä instead of Jäätä muodostuu järvelle?

This sentence follows the classic existential word order: Location + Verb + Subject. When introducing new information, such as ice appearing, Finnish naturally puts the location first and the new subject at the end.