Learn Hardcore Finnish: Matkalla kotiin poikkean kauppaan, koska tarvitsen vaihtorahaa. - On the way home I stop at the store because I need change.

Matkalla is the noun matka (trip/journey) in the adessive case (-lla/-llä), which often corresponds to meanings like on, at, or during something.
So matkalla here means while on the way / during the journey (literally something like on a trip).
A common pattern is olla matkalla = to be on the way.

Why is it kotiin and not koti or kotona?

Because the sentence expresses movement toward home.

kotiin = to home (the illative case, “into/toward”)
koti = home (basic form, not showing direction/location by itself)
kotona = at home (the essive/adessive? actually adessive: -na/-nä is essive, -lla/-llä is adessive; kotona is inessive? Correction: kotona is the essive? No—kotona is essive-like historically, but synchronically it functions as a locative “at home”; learners can treat it as a fixed form meaning at home.)
In practical terms: kotiin answers where to?

Is Matkalla kotiin a fixed phrase? Could I say it differently?

It’s a very common and natural phrase meaning on the way home. Other natural options include:

Kotimatkalla = on the way home (a compound: koti
- matkalla)
Kun olen matkalla kotiin, ... = When I’m on my way home, ... (more explicit)
Kotiin päin = towards home (more “in the direction of”)

Why is there no word for I (minä)?

Finnish usually drops subject pronouns because the verb ending already shows the person.
poikkea-n = I stop by (1st person singular)
tarvitse-n = I need (1st person singular)
You can add minä, but it adds emphasis/contrast: Minä poikkean... = I (not someone else) am stopping by...

What does poikkean mean exactly, and why use it instead of käyn?

poiketa means to deviate/stop by briefly—it has the nuance of making a small detour.
So poikkean kauppaan implies I’ll pop into the shop (on the way), not as the main trip.

Compared with:

käyn kaupassa = I go to the shop / I do shopping / I visit the shop (neutral, very common)
pistäydyn kaupassa = I pop in (also “brief visit,” very colloquial)

Why is it kauppaan (with -aan)?

kauppaan is kauppa (shop/store) in the illative case, showing movement into/to the shop.
The illative often looks like:

talo → taloon (into the house)
kauppa → kauppaan (into the shop)

So poikkean kauppaan = I stop by to the shop / pop into the shop.

Could it be poikkean kaupassa instead?

Yes, but the meaning shifts slightly depending on what you want to emphasize:

poikkean kauppaan emphasizes going into the shop (direction/entry)
poikkean kaupassa emphasizes being at the shop briefly (location during the visit)

Both can be correct; kauppaan is very natural when the focus is “I’m going there (as a stop).”

Why is the word order Matkalla kotiin poikkean kauppaan and not something like Poikkean kauppaan matkalla kotiin?

Finnish word order is flexible, and word order often reflects what you frame as the context/topic first.
Starting with Matkalla kotiin sets the scene: As for my way home...
You can also say:

Poikkean matkalla kotiin kauppaan... (still fine)
Poikkean kauppaan matkalla kotiin... (fine; puts more emphasis on the action first)

The chosen order here sounds natural and “narrative”: context → action → reason.

What does koska do, and is it different from sillä?

koska means because and introduces a reason clause: ..., koska tarvitsen...
Compared with:

koska = because (neutral, very common in speech and writing)
sillä = for / because (more explanatory, often more formal or “writerly”)

In many everyday sentences, koska is the default.

Why is it tarvitsen (present tense)? Shouldn’t it be future: “I will need”?

Finnish often uses the present tense to talk about near-future plans and reasons, especially when it’s obvious from context.
So tarvitsen vaihtorahaa can cover I need / I’m going to need depending on context. The time idea comes from the situation, not necessarily a future tense form (Finnish doesn’t have a dedicated future tense).

Why is vaihtorahaa in the partitive (ending -a)?

vaihtoraha means change (money), and here it’s treated as an uncountable/indefinite amount: some change. Finnish commonly uses the partitive for:

an unspecified quantity of something
mass nouns (like money, water, coffee) So tarvitsen vaihtorahaa ≈ I need some change.

If you had a specific amount, you might use something else, e.g. Tarvitsen viisi euroa vaihtorahaa (I need five euros in change).

What exactly is vaihtoraha? Is it one word or two?

It’s a compound noun: vaihto (exchange/change) + raha (money) → vaihtoraha = change (coins/small money used for paying).
In the sentence, it appears in the partitive: vaihtorahaa.

Why is there a comma before koska?

In standard Finnish punctuation, you usually put a comma before a subordinate clause starting with conjunctions like koska (because), että (that), kun (when), etc.
So: ..., koska tarvitsen vaihtorahaa. is the normal written style.