Learn Finnish: Pohtiessani elämääni opin paljon. - While pondering my life, I learn a lot.

They have been dropped! In standard Finnish, you can (and usually do) drop the 1st and 2nd person pronouns (minä, sinä, me, te) and their genitive forms (minun, etc.) because the endings do all the work. Here, the -ni on pohtiessani and elämääni tells us it's "my", and the -n on opin tells us it's "I".

What grammatical structure is pohtiessani, and why not just say kun pohdin?

Pohtiessani is a temporal construction (often called the tehdessä structure), built from the second infinitive of pohtia. It translates to "while I ponder." It acts as a shorter, more elegant alternative to saying kun pohdin (when I ponder). The possessive suffix -ni at the end is what tells us who is doing the pondering.

Why is elämääni in the partitive case?

The verb pohtia (to ponder, think about) is a partitive verb. It expresses an ongoing action without a clear start and finish, so its object must be in the partitive. The base word elämä (life) takes the partitive ending -ä to become elämää, and then the possessive suffix -ni is tacked onto the very end to make elämääni.

Would it be wrong to write ?