Learn Finnish: Teen listan ennen kauppaa. - I make a list before the shop.

When you use verbs of creation like tehdä, using the total object (listan) means you intend to finish making the whole list. If you used the partitive (listaa), it would imply you are just in the middle of making it.

I thought Finnish mostly uses postpositions. Why is ennen placed before kauppaa?

While most Finnish position words go after the noun, ennen is one of the few true prepositions. It almost always comes before the word it modifies.

Why is kauppa in the partitive form (kauppaa) here?

The preposition ennen always requires the word following it to be in the partitive case. So, kauppa becomes kauppaa.

Does kauppa refer to the physical shop building here?

While kauppa literally means 'shop' or 'store', in everyday phrases like this it refers to the activity of shopping. So ennen kauppaa is naturally understood as 'before going shopping'.