Learn Croatian: Tajnica zna sve o tvrtki. - The secretary knows everything about the company.

You are thinking of a sound change called sibilarization, which normally changes 'k' to 'c' (like slika becoming slici). However, Croatian often skips this rule if it would create a hard-to-pronounce consonant cluster. For nouns ending in -tka (like tvrtka or patka), the 'k' usually stays the same in the dative and locative cases, making it tvrtki.

Which case is tvrtki in, and what triggers it?

It is in the locative case. The preposition o (meaning "about" or "concerning") requires the locative case when you are talking about the subject of a thought, conversation, or knowledge.

Why do we use sve here instead of sav or svi?

The word sve is the neuter singular form of the word sav (all/whole). When used on its own as a pronoun without a noun attached to it, the neuter singular sve translates to the general concept of "everything". In contrast, svi means "everyone" or "all" for masculine plural nouns.

Are there other common words for "company" besides tvrtka?