Learn Finnish: On luonnollista säpsähtää, kun kuuluu kova ääni. - It is natural to startle when a loud noise is heard.

The base verb is säpsyä, which means to twitch, startle, or be generally jumpy. Adding the momentane suffix -ähtää creates säpsähtää, which restricts the action to happening exactly once, very suddenly and briefly. It translates perfectly to giving a single, sudden flinch or start.

Why is the adjective luonnollista in the partitive case instead of the nominative luonnollinen?

This sentence uses the "It is [adjective] to do [something]" structure. In Finnish, when an adjective describes an abstract concept or an action (like the infinitive verb säpsähtää) rather than a concrete noun, the adjective takes the partitive case.

The translation says "is heard." Why does Finnish use the active verb kuuluu instead of the passive kuullaan?

The verb kuulua is inherently intransitive and means "to be audible" or "to sound." Finnish prefers this active structure to simply state that a noise can be heard. Using the passive kuullaan would strongly imply that an unspecified group of people is actively listening to it.

Why is the word order kuuluu kova ääni (verb-subject) rather than kova ääni kuuluu?

This is an existential clause, which introduces new things into a scene. Because "a loud noise" is indefinite and new information to the listener, Finnish puts the verb first and places the new subject at the end of the clause.