Learn Finnish: Piilotin mikrofonin huoneeseen kuullakseni kaiken. - I hid the microphone in the room to hear everything.

This is the structure you are currently learning! It is made of the basic dictionary form of the verb kuulla (to hear) + the translative ending -kse- + the first-person singular possessive suffix -ni (my). Together, they form the meaning "in order for me to hear".

What would happen to kuullakseni if the subject of the sentence was "he" or "she"?

The possessive suffix at the end of this "in order to" structure must always match the person doing the action. If the main verb was piilotti (he/she hid), the suffix would change to the third-person suffix, making the word kuullakseen (in order for him/her to hear).

Why do we use huoneeseen (into the room) when the English translation says "in the room"?

In Finnish, the verb piilottaa (to hide) is treated as a verb of movement because you are moving an object into a hiding place. Therefore, it requires a directional case like the illative -seen (into) rather than a static location case like the inessive -ssa (in).

Why is the word for "everything" in the form kaiken instead of the basic form kaikki?

The word kaikki is the direct object of the verb kuulla. Even though kuullakseni is an infinitive structure, the main sentence has a clear personal subject ("I" hid). Because of this, the object follows standard object rules and takes the genitive-looking accusative case kaiken.