Questions & Answers about Heidän setänsä on opettaja.
Why do we need the -nsä ending on setänsä if we already have heidän to mean "their"?
In standard Finnish, the third-person pronouns hänen (his/her) and heidän (their) require the noun to also take a possessive suffix. You need both pieces to be grammatically correct, so setä (uncle) becomes setänsä.
Why is the suffix -nsä instead of -nsa?
This is due to vowel harmony. The word setä contains the front vowels e and ä. Because it has front vowels, it must take the front-vowel version of the suffix (-nsä) instead of the back-vowel version (-nsa).
Can I leave out the word heidän and just say "Setänsä on opettaja"?
No, you usually cannot drop hänen or heidän. While you can often drop the pronoun for "my" or "your" (like saying setäni instead of minun setäni), third-person pronouns like heidän are required unless the "owner" is already the subject of the same sentence.
How do I know if setänsä means "his/her uncle" or "their uncle"?