Learn Finnish: En ymmärrä omaa alitajuntaani, sinun alitajunnastasi puhumattakaan. - I do not understand my own subconscious, not to mention your subconscious.

It means "let alone" or "not to mention." It is formed using the third infinitive abessive of puhua (to speak), which is puhumatta (without speaking), plus the clitic -kaan (even/also not). Literally, it translates to "even without speaking of."

Why is alitajunnastasi in the elative (-sta) case?

It is governed by the root verb inside puhumattakaan. The verb puhua requires the elative case (-sta/-stä) when you talk about something. So, alitajunnastasi puhumattakaan structurally means "even without speaking about your subconscious."

Why is omaa alitajuntaani in the partitive case?

It is the direct object of a negative verb (en ymmärrä). In Finnish, negative sentences force the object into the partitive case. The base word is alitajunta, its partitive is alitajuntaa, and adding the possessive suffix -ni gives us alitajuntaani.

The pronoun sinun is used, but minun is omitted before omaa. Why?