Kirjoitan kauppalistan paperille ennen kuin menen lähikauppaan.

Because kauppalistan is the object of kirjoitan and here it is a total object: the idea is that the whole shopping list gets written.

In Finnish, a singular total object often looks like the genitive, so:

• kauppalista = shopping list
• kauppalistan = the shopping list / a complete shopping list as the object

Compare:

• Kirjoitan kauppalistan. = I write the shopping list / I will write the whole list.
• Kirjoitan kauppalistaa. = I am writing a shopping list / I write some of the list / the action is incomplete or ongoing.

So kauppalistan suggests a complete, bounded result.

Paperille is the allative case, which often means onto or to a surface.

Here the idea is writing onto paper, so paperille is the natural choice.

• paperi = paper
• paperille = onto the paper / onto paper

Why not the others?

• paperilla = on paper, on the paper
  This describes location or state more than direction. It can work in some contexts, but with kirjoittaa Finnish very often uses paperille because the writing is being put onto the surface.
• paperiin = into the paper
  This sounds wrong here, because you are not going into the paper.

So:

• Kirjoitan paperille. = I write on/onto paper.

Lähikauppaan is in the illative case, which often means into or to a place.

• lähikauppa = local shop / nearby grocery store
• lähikauppaan = to the local shop / into the local shop

Because the verb mennä means to go, Finnish uses a directional case here:

• menen lähikauppaan = I go to the local shop

This is very common with places:

• menen kouluun = I go to school
• menen kotiin = I go home
• menen kauppaan = I go to the shop

Ennen kuin means before in the sense of before something happens.

It introduces a whole clause with its own verb:

• ennen kuin menen lähikauppaan = before I go to the local shop

This is different from just ennen, which can also be used with nouns or noun-like expressions:

• ennen kauppaa = before the shop / before shopping
• ennen lähtöä = before leaving

But when you want before + subject + verb, Finnish uses ennen kuin:

• ennen kuin syön = before I eat
• ennen kuin lähden = before I leave

Finnish often leaves out the subject pronoun when it is already clear from the verb ending.

• menen already means I go
• kirjoitan already means I write

So minä is not necessary.

This is very normal Finnish style:

• Kirjoitan = I write
• Menen = I go

You can include minä for emphasis, contrast, or clarity:

• Minä menen lähikauppaan, mutta hän menee markettiin.
  = I’m going to the local shop, but he/she is going to the supermarket.

In your sentence, leaving out minä sounds natural and neutral.

Finnish often uses the present tense to talk about the near future or planned future, especially when the context makes the time clear.

So:

• Kirjoitan kauppalistan... can mean I write or I will write
• ennen kuin menen lähikauppaan can mean before I go or before I will go

English often prefers a future-like meaning in translation, but Finnish usually does not need a separate future tense.

This is very common:

• Menen huomenna kauppaan. = I’m going / I will go to the shop tomorrow.
• Soitan sinulle myöhemmin. = I’ll call you later.

So the present tense here is completely normal.

Lähikauppa literally means something like near shop or local shop.

It usually refers to a nearby grocery store or convenience-type store in your neighborhood.

It is a compound word:

• lähi- = near, local
• kauppa = shop, store

So:

• kauppaan = to the shop/store
• lähikauppaan = to the local shop / nearby store

A native English speaker may be tempted to translate it word-for-word, but in natural English it is often best understood as the local shop, the nearby store, or sometimes the neighborhood grocery store.

Yes, the parts can be switched.

The given order is natural because it starts with the main action:

• Kirjoitan kauppalistan paperille ennen kuin menen lähikauppaan.

But Finnish word order is quite flexible. You could also say:

• Ennen kuin menen lähikauppaan, kirjoitan kauppalistan paperille.

That puts more focus on the time relationship first: Before I go to the local shop...

So the original order is not the only possible one; it is just a neutral and natural choice.

Finnish does not have articles like English a/an and the.

So a noun like kauppalista or lähikauppa does not automatically show whether it means:

• a shopping list
• the shopping list

or whether lähikauppa means:

• a local shop
• the local shop

You understand that from context.

In this sentence:

• kauppalistan could be understood as a shopping list or the shopping list
• lähikauppaan could mean to the local shop or to a local shop, though context often makes the local shop more likely

This is one of the biggest differences from English.

Yes, literally kauppalista is made from:

• kauppa = shop / store / commerce
• lista = list

But as a compound noun, kauppalista means shopping list.

This is very common in Finnish: compounds are often understood by usage, not just word-for-word logic.

So even though kauppa usually means shop/store, the compound kauppalista refers to a list of things to buy.

Not always, but very often when you mean writing something down onto a surface.

Examples:

• Kirjoitan nimen paperille. = I write the name on the paper.
• Kirjoitin numeron seinälle. = I wrote the number on the wall.
• Kirjoita vastaus taululle. = Write the answer on the board.

You can also use kirjoittaa without mentioning where:

• Kirjoitan kauppalistan. = I’m writing a shopping list.

Or with another expression:

• Kirjoitan tietokoneella. = I write on a computer / with a computer.

So paperille is not required, but in this sentence it specifies the medium very naturally.

Because Finnish needs ennen kuin when before introduces a full clause with a finite verb.

A clause like menen lähikauppaan has a subject built into the verb and its own finite verb, so kuin is required:

• ennen kuin menen lähikauppaan

You cannot say ennen menen lähikauppaan.

But if there is no full clause, then ennen by itself can be enough:

• ennen kauppareissua = before the shopping trip
• ennen lähtöä = before leaving

So the rule is roughly:

• ennen + noun/expression
• ennen kuin + clause

It can mean either, depending on context.

By itself, the sentence can mean:

• a habitual action: I write a shopping list on paper before I go to the local shop
• a planned/near-future action: I’ll write a shopping list on paper before I go to the local shop

Finnish present tense often allows both readings.

If you wanted to make the habitual meaning clearer, context words could help:

• Yleensä kirjoitan kauppalistan paperille ennen kuin menen lähikauppaan.
  = I usually write a shopping list on paper before I go to the local shop.

If you wanted a more specific future reading, context could also help:

• Tänään kirjoitan kauppalistan paperille ennen kuin menen lähikauppaan.
  = Today I’ll write a shopping list on paper before I go to the local shop.