Learn Hardcore Finnish: Vaikka olen jalankulkija, muistutan lapsia myös siitä, että turvavyö on pakko laittaa, kun he menevät kaverin autoon. - Although I am a pedestrian, I also remind the children that the seat belt must be put on when they go into a friend’s car.

Olen jalankulkija = I am a pedestrian (a simple statement of identity or role).
Olen jalankulkijana = As a pedestrian / In my role as a pedestrian (emphasizes the role or perspective).

In this sentence, Vaikka olen jalankulkija stresses a general, permanent fact about the speaker: they do not usually drive; they are a pedestrian. Using jalankulkijana would shift the focus to “in my capacity as a pedestrian”, which is stylistically a bit heavier and less neutral here.

So olen jalankulkija is the natural, default way to say I am a pedestrian.

Why is lapsia in the partitive plural instead of lapset?

The verb muistuttaa typically takes the object in the partitive when you “remind someone (of something):

muistuttaa jotakuta jostakin
= remind someone of something

So:

lapsia is the partitive plural of lapset.
It corresponds to children as the people being reminded.

Reasons for lapsia instead of lapset:

Verb pattern:
With muistuttaa, the person reminded is naturally in the partitive.
Indefiniteness:
lapsia often suggests an indefinite or not-fully-specified group of children (“children in general” / “the kids I deal with”), not a closed, clearly delimited group.
muistutan lapset would sound like “I remind the children” (a specific, complete group) and is much less natural with muistuttaa in this sense.

So muistutan lapsia is the standard way to say I remind children here.

What is going on in muistutan lapsia myös siitä, että ...? Why siitä and why the comma before että?

The verb muistuttaa has the pattern:

muistuttaa
- (partitive) person
  - jostakin / siitä, että...
    - muistutan lapsia jostakin = I remind the children about something
    - muistutan lapsia siitä, että... = I remind the children that...

siitä:

siitä is the elative form of se (“from that” / “about that”).
With muistuttaa, the thing you remind someone about is in the elative: jostakin / siitä.
Here siitä is a pronoun that “points forward” to the entire että-clause:
- siitä, että turvavyö on pakko laittaa
  = about the fact that the seatbelt must be fastened.

Comma before että:

In Finnish, you normally put a comma between a main clause and a subordinate clause, including että-clauses.
So: muistutan lapsia myös siitä, että ...
Main clause: muistutan lapsia myös siitä
Subordinate: että turvavyö on pakko laittaa...

Even though in English you wouldn’t usually put a comma before that, in Finnish the comma is required here.

What does turvavyö on pakko laittaa literally mean, and why this structure instead of a normal “must” verb?

Literally, turvavyö on pakko laittaa is:

turvavyö = seat belt
on = is
pakko = necessity / compulsion
laittaa = to put (on)

So literally: “the seat belt is (a) necessity to put (on)”.

Functionally, olla pakko + infinitive means must / have to:

Minun on pakko lähteä. = I must go.
On pakko laittaa turvavyö. = You must fasten the seat belt.

In this sentence, the structure is slightly re-ordered:

turvavyö on pakko laittaa
= the seat belt has to be put on / it’s mandatory to put on the seat belt.

It’s a very common idiomatic way in Finnish to express obligation, instead of using a single modal verb like English must.

Why is the verb laittaa used for a seat belt? Does it mean “put” or “fasten”?

Laittaa is a very general verb meaning to put / to place / to set. In many contexts it can overlap with panna.

With clothes or equipment, laittaa often means to put on / to wear / to fasten:

laittaa päälle takki = put a coat on
laittaa kengät jalkaan = put shoes on
laittaa turvavyö (or laittaa turvavyö päälle) = fasten the seat belt

So in this sentence, turvavyö on pakko laittaa is best understood as “the seat belt has to be fastened / put on”, even though the literal verb is just “put”.

Why is it kun he menevät kaverin autoon and not kaverin autossa? What does the case -oon express?

The ending -oon on autoon is the illative case, marking movement into something.

auto = car (basic form)
autoon = into the car

Contrast:

autossa (inessive) = in the car (location, no movement)
autoon (illative) = into the car (movement towards/into)

The verb mennä (to go) naturally combines with a destination case:

mennä kouluun = go to (into) school
mennä kotiin = go home
mennä autoon = go into the car

So kun he menevät kaverin autoon = when they go into a friend’s car.
Using autossa would describe being already inside the car, not the movement into it.

Why is it kaverin autoon and not just kaveri auto or something similar? What does kaverin do here?

Kaverin is the genitive singular of kaveri (“friend / buddy”).

In Finnish, possession is expressed by putting the possessor in the genitive before the possessed noun:

kaverin auto = the friend’s car
äidin auto = mother’s car
lapsen kirja = the child’s book

Here:

kaverin autoon = into the friend’s car
- kaverin (genitive) = of the friend
- autoon (illative) = into the car

So kaverin is simply marking that the car belongs to the friend. There is no article “a/the” in Finnish; this genitive construction is how you show “friend’s car”.

Why is he menevät used for “children”? Could it also be ne menevät?

In standard written Finnish:

he = they (for people)
ne = they (for things / animals, traditionally non-human)

So talking about children, standard language uses he:

lapset → he menevät = they go

In colloquial spoken Finnish, people often use ne for people too:

Ne menee kaverin autoon. (spoken)
= He menevät kaverin autoon. (standard)

But in a neutral written sentence like this, he menevät is the correct and expected form.

Why is myös placed after lapsia: muistutan lapsia myös siitä…? Could it be somewhere else?

Myös means also / too and is fairly flexible in word order, but its position affects what is emphasized.

muistutan myös lapsia siitä...
Emphasis: I also remind the children (in addition to reminding someone else).
muistutan lapsia myös siitä, että...
Emphasis: I remind the children also about that (in addition to other things).

In the given sentence, the focus is on the thing they are being reminded about (seat belts), not on who is reminded. So myös is placed close to siitä, the “thing”:

I remind children also of this fact: that the seat belt must be fastened.

So yes, you could move myös, but the nuance changes:

Vaikka olen jalankulkija, myös muistutan lapsia siitä... – ungrammatical / very odd.
Vaikka olen jalankulkija, muistutan myös lapsia siitä... – suggests there are others besides children that I remind.
Vaikka olen jalankulkija, muistutan lapsia myös siitä... – I remind children, and this (seat belt) is also one of the things I remind them about.

Is siitä really necessary? Could you just say muistutan lapsia myös, että turvavyö on pakko laittaa?

You can sometimes hear or see muistuttaa used directly with an että-clause, but the most natural and clearly grammatical pattern is:

muistuttaa jotakuta siitä, että...

So:

muistutan lapsia myös siitä, että turvavyö on pakko laittaa
is the standard, fully natural form.

If you drop siitä:

muistutan lapsia myös, että turvavyö on pakko laittaa

this becomes less standard and can feel a bit incomplete or spoken-like. Some speakers might accept it, but grammatically and stylistically, siitä, että is the recommended construction.

Rule of thumb: with muistuttaa meaning remind, it’s safest to keep siitä, että....

What does jalankulkija literally mean? Is it a compound word?

Yes, jalankulkija is a compound:

jalan = on foot (from jalka, “foot / leg”, in a form that here means “by foot”)
kulkija = walker / goer / traveler (from kulkea, “to go / to move / to travel on foot”)

So jalankulkija literally means something like “one who goes by foot”, i.e. a pedestrian.