Learn Finnish: Viritän ansan hyvällä syötillä. - I set the trap with a good bait.

Because setting the trap is seen as a completed action, requiring a total object. The genitive-like accusative ending -n shows that you will successfully finish setting the trap. If you were currently in the middle of setting it or doing it without a definite end, you would use the partitive ansaa.

Why does 'with a good bait' use the adessive case (hyvällä syötillä)?

In Finnish, the adessive case (ending in -lla or -llä) is heavily used to express the instrument or tool used to perform an action. Even though it often translates to physical location ('on' or 'at'), here it indicates the means by which the trap is set.

Both viritän and syötillä have a single 't'. What are their base forms?

The base forms are the verb virittää (to set/tune) and the noun syötti (bait). Both undergo quantitative consonant gradation (tt weakens to t) because adding the first-person ending (-n) and the adessive case ending (-llä) closes their final syllables.

How would I adapt this sentence to use the current lesson's vaikka kuinka + conditional structure?