Questions & Answers about Heidän ongelmansa on pahentunut.
How does the -tua suffix change the meaning of the verb in pahentunut?
The verb pahentua (to get worse) is derived from the adjective paha (bad). While pahentaa means to make something else worse, adding the automative -tua suffix means the action happens by itself. The problem didn't make something else worse; it just got worse on its own.
Why do we need the -nsa ending on ongelmansa if we already have heidän?
In standard Finnish, when you use the third-person genitive pronouns hänen (his/her) or heidän (their), you must also attach the matching possessive suffix to the noun. So, "their problem" becomes heidän ongelmansa. In everyday spoken Finnish, you will often hear people drop the suffix and just say niiden ongelma.
Why is the verb in the form on pahentunut rather than pahentui?
The sentence uses the perfect tense ("has worsened") to show that the worsening started in the past and is still relevant to the present situation. It is formed with the auxiliary verb olla (here, on) and the past participle of the main verb (pahentunut). If you used the simple past pahentui, it would simply mean "worsened".
Since heidän means "their", why is the auxiliary verb singular (on) instead of plural (ovat)?