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Questions & Answers about Rikoitko lukkosi tahallaan?
Where did the word for "you" (sinä) go in this sentence?
It was dropped! Because the verb ending in rikoit already tells us it means "you broke", the pronoun sinä is often left out. The question particle -ko is just attached straight to the verb, making Rikoitko ("Did you break?").
Where is the word sinun (your) before lukkosi?
Just like with subject pronouns, Finnish frequently drops the possessive pronouns minun (my) and sinun (your). Because the suffix -si on the end of lukkosi already perfectly means "your", adding sinun is optional and usually left out.
If I completely broke the lock, shouldn't the object be in the genitive-accusative case, like lukon?
It actually is! The base form of the object here is indeed lukon. However, Finnish has a strict rule: the -n ending disappears when you add a possessive suffix. So, lukon + -si merges to become lukkosi, rather than the incorrect lukonsi.
I see tahallaan means "on purpose". Since the sentence is about "you", shouldn't it be tahallasi?