Questions & Answers about Jouw kettinkje is mooi, maar erg duur.
How does the word ketting change into the diminutive kettinkje?
The base word for necklace is ketting. For words ending in an unstressed -ing, the Dutch diminutive rule says we drop the -g, replace it with a -k, and add -je. This makes it easier to pronounce, turning ketting into kettinkje.
Why use jouw here instead of the shorter je for 'your'?
Jouw is the stressed, emphatic version of je. You use jouw when you want to put special focus on the owner, as if saying 'Your little necklace is beautiful (as opposed to someone else's)'.
I thought erg meant 'bad'. Why does it mean 'very' in this sentence?
You are right that as a normal adjective, erg means 'bad' or 'terrible'. But when you put it directly in front of another adjective (like duur), it acts as an intensifier meaning 'very' or 'really'. You could also just say heel duur here to mean the exact same thing.
Does the word maar (but) change the word order of the sentence?
No, maar is a connecting word (a coordinating conjunction) that links two sentences together without mixing up their word order. The second part of this sentence is just a shortened way of saying maar het is erg duur, keeping the normal subject-verb order.