Learn Hardcore Finnish: Autossa tärkein liikennesääntö on, että turvavyö laitetaan kiinni aina ennen lähtöä. - In the car the most important traffic rule is that the seat belt is fastened always before departure.

In Autossa tärkein liikennesääntö on..., what does autossa literally mean, and what case is it in? Is autossa the subject of the sentence?

Autossa is in the inessive case (ending -ssa/-ssä), which usually means “in” something.

auto = car
autossa = in the car

So autossa literally means “in the car.”

It is not the subject of the sentence. The structure is:

Autossa = adverbial (location: in the car)
tärkein liikennesääntö = the subject (the most important traffic rule)
on = the verb “is”

So the core sentence is: Tärkein liikennesääntö on, että...
Adding Autossa at the beginning just sets the context: In the car, the most important traffic rule is that...

How does tärkeä become tärkein? What form is tärkein?

Tärkein is the superlative form of the adjective tärkeä (important).

The degrees are:

tärkeä = important (positive)
tärkeämpi = more important (comparative)
tärkein = most important (superlative)

So tärkein liikennesääntö = “the most important traffic rule.”

Notice there is no separate word for “the” in Finnish; the superlative form plus context usually gives the meaning “the most important …” rather than just “a most important …”.

What exactly does liikennesääntö mean, and how is this word formed?

Liikennesääntö is a compound noun:

liikenne = traffic
sääntö = rule

Put together: liikennesääntö = “traffic rule”.

In Finnish, compounds are very common. You simply write the parts together as one word:

liikenne
- valo → liikennevalo (traffic light)
turva
- vyö → turvavyö (safety belt / seat belt)

In this sentence, liikennesääntö is in the nominative singular (basic dictionary form) and acts as part of the subject: tärkein liikennesääntö.

Why is there a comma before että in on, että turvavyö laitetaan kiinni? How does että work here?

Että is a subordinating conjunction meaning roughly “that” in English (as in “I think that…”).

The structure is:

(Se) tärkein liikennesääntö on, että …
= The most important traffic rule *is that …*

In standard written Finnish, you put a comma before että when it introduces a content clause (a subordinate clause that functions like “that + sentence” in English).

So:

On tärkeää, että tulet ajoissa.
= It is important that you come on time.

Same pattern in the example sentence.

What kind of verb form is laitetaan? Why isn’t it something like laitamme or laitat?

Laitetaan is the impersonal / passive form of the verb laittaa in the present tense.

laittaa = to put (on), to place
laitetaan = is put (on), or one puts (on), or general “you/people put (on)”

This impersonal form:

does not have an explicit subject (no “we/you/they”)
describes a general rule, habit, or action that people in general do

So turvavyö laitetaan kiinni can be understood as:

“the seat belt is fastened” (passive English)
or
“you/one fastens the seat belt” (general rule)

Using laitamme (we put on) or laitat (you put on) would make it about a specific “we” or “you”, not a general traffic rule.

Where is the subject of laitetaan? Who is doing the action in turvavyö laitetaan kiinni?

In Finnish impersonal (passive) forms like laitetaan, there is no explicit subject.

The pattern:

turvavyö = grammatical object (the thing affected)
laitetaan = impersonal verb (someone/people do it)
kiinni = how it is done (fastened/closed)

So turvavyö laitetaan kiinni literally is more like:

“The seat belt is put fastened”
with the doer being “people in general / one / you” understood from context, but not stated.

This is very typical in Finnish rules, instructions, and general statements.

Why is turvavyö singular in Finnish, while in English we often talk about “seat belts” in the plural?

Turvavyö is singular:

turva = safety
vyö = belt
turvavyö = a safety belt / seat belt

In Finnish, it’s common to talk about one generic seat belt when describing a rule, even if cars have more than one. It’s like saying:

“The (each) seat belt must be fastened.”

You can use the plural turvavyöt, but in this kind of general rule, the singular is very natural: it expresses the idea “the seat belt (in principle) must be fastened.”

What does kiinni mean in turvavyö laitetaan kiinni? Why do we need both laitetaan and kiinni?

Kiinni is an adverb/particle that roughly means:

“closed”, “shut”, “fastened”, “attached”

The verb phrase laittaa kiinni is a common combination meaning “to close / to fasten / to shut”, depending on the object:

laittaa oven kiinni = to close the door
laittaa ikkunan kiinni = to close the window
laittaa turvavyö kiinni = to fasten the seat belt

So:

turvavyö laitetaan kiinni = the seat belt is fastened
(literally: the seat belt is put fastened/closed)

You need both:
laitetaan tells you there is an action of putting/doing;
kiinni specifies how (into the “closed/fastened” state).

What does aina do in the sentence, and can it be placed in a different position?

Aina means “always.”

In the sentence:

turvavyö laitetaan kiinni aina ennen lähtöä
= the seat belt is always fastened before departure

Typical positions:

Turvavyö laitetaan aina kiinni ennen lähtöä.
Turvavyö laitetaan kiinni aina ennen lähtöä.

Both are fine; word order is fairly flexible, and the meaning doesn’t change much here.
Moving aina too far away from the verb or time expression can sound a bit unnatural, but within that verb phrase area it’s flexible.

Why is it ennen lähtöä and not something like ennen lähtö or ennen lähdön?

Ennen is a preposition meaning “before.”
In Finnish, ennen always takes the noun in the partitive case.

lähtö = departure (nominative)
lähtöä = partitive form of lähtö

So the rule is:

ennen + partitive → ennen lähtöä = before the departure / before leaving

Ennen lähtö is simply wrong.
Ennen lähdön (genitive) is also not standard; ennen specifically triggers the partitive.

Can we say ennen kuin lähdetään instead of ennen lähtöä? What’s the difference?

Yes, you can say:

…turvavyö laitetaan kiinni aina ennen kuin lähdetään.

Comparison:

ennen lähtöä
= before the departure / before leaving (noun phrase)
ennen kuin lähdetään
= before (we/you/they) leave (full clause with a verb)

Both express almost the same idea.
Ennen lähtöä is a bit more compact and nominal;
ennen kuin lähdetään makes the action (leave) explicit and uses the same impersonal form (lähdetään) as laitetaan.

Does Finnish have a word for “the” in “the most important traffic rule”, or how is definiteness expressed in tärkein liikennesääntö?

Finnish has no articles like “the” or “a/an.”
Definiteness is usually understood from:

context,
word form (like superlatives), and
word order.

Tärkein liikennesääntö by itself can theoretically mean:

a most important traffic rule
or
the most important traffic rule

In practice, in a sentence like Autossa tärkein liikennesääntö on, että…, the natural interpretation is “the most important traffic rule in the car is that...” because we are clearly talking about a specific “most important” rule in that context. The superlative strongly suggests a definite meaning here, even without any word for “the.”