Learn Finnish: Menen observatorioon. Kaukoputkeni on nimittäin rikki. - I am going to the observatory. My telescope is broken, you see.

Unlike conjunctions like koska (because), nimittäin acts as an explanatory particle rather than a subordinating conjunction. It is typically placed in a delayed position, right after the first main element of its clause. Here, that first element is the subject and verb phrase Kaukoputkeni on, so nimittäin tucks in immediately after them.

What is the difference in tone between using nimittäin here versus using koska?

Using koska would merge this into a single sentence: Menen observatorioon, koska kaukoputkeni on rikki. Using nimittäin splits the thought into two independent sentences, making it sound more conversational and naturally explanatory, exactly like adding "you see" or "namely" to an English sentence.

Why is the word for "my telescope" kaukoputkeni instead of kaukoputkini?

Possessive suffixes like -ni attach to a word's inflectional stem, not its dictionary form. The word kaukoputki (telescope) has the stem kaukoputke- (which you can also see in its genitive form, kaukoputken). Adding -ni to this stem results in kaukoputkeni.

Does the word rikki (broken) change if the subject is plural or in a different case?