Learn Hardcore Finnish: Risteyksessä on paljon jännitystä, kun joku yrittää ohittaa auton liian nopeasti. - There is a lot of tension at the intersection when someone tries to overtake a car too fast.

So risteyssä is not possible, because the consonant cluster ks is part of the stem, not part of the ending. You'll get the same pattern in other -ys words:

yhdistys → yhdistyksessä (in the association)
järjestys → järjestyksessä (in order)

What does the ending -ssä in risteyksessä mean?

The ending -ssa / -ssä is the inessive case, usually translated as in or at:

talossa = in the house
bussissa = on / in the bus
kaupungissa = in the city
risteyksessä = at the intersection / in the intersection

So risteyksessä on paljon jännitystä literally:
In the intersection there is a lot of tension.

Could you also say risteyksellä instead of risteyksessä?

You can, but the nuance is slightly different.

risteyksessä (inessive -ssa/-ssä) focuses on being in the intersection area, inside the crossing itself.
risteyksellä (adessive -lla/-llä) is more like at the intersection / at the junction, often from a bit more external point of view.

In practice, risteyksessä is the most natural choice for traffic situations like this sentence. Risteyksellä wouldn’t be wrong, but it’s less typical here.

Why is it paljon jännitystä and not paljon jännitys?

Because paljon (a lot, much) requires the partitive case after it.

jännitys = tension (nominative)
jännitystä = tension (partitive)

After paljon, the noun always goes into partitive:

paljon vettä = a lot of water
paljon ihmisiä = a lot of people
paljon jännitystä = a lot of tension

So paljon jännitys is ungrammatical; it must be paljon jännitystä.

Why is jännitystä in the partitive, not just jännitys?

There are two reasons:

The word paljon
As noted above, paljon always takes a partitive noun: paljon jännitystä.
Type of meaning
Jännitys is something you usually talk about as an amount or degree, not as a single countable item. For uncountable or “mass” concepts in existential sentences, Finnish frequently uses the partitive.

So Risteyksessä on paljon jännitystä =
There is a lot of tension at the intersection.

Using nominative jännitys here would sound wrong.

Why is the word order Risteyksessä on paljon jännitystä, not Paljon jännitystä on risteyksessä like in English?

Finnish commonly uses an existential sentence structure:

[Place] + on + [something]

This structure is used where English has “There is/are …”:

Pöydällä on kirja. = There is a book on the table.
Kadulla on paljon autoja. = There are many cars on the street.
Risteyksessä on paljon jännitystä. = There is a lot of tension at the intersection.

The location (risteyksessä) comes first, then the verb on, then what exists there (paljon jännitystä).

You could say Paljon jännitystä on risteyksessä, but it sounds marked and unusual in this context, as if you were strongly emphasizing paljon jännitystä for some special reason.

Why is there a comma before kun?

In written Finnish, subordinate clauses introduced by conjunctions like kun, koska, jos, että are usually separated with a comma from the main clause.

Here:

Main clause: Risteyksessä on paljon jännitystä
Subordinate clause: kun joku yrittää ohittaa auton liian nopeasti

So a comma is standard:
Risteyksessä on paljon jännitystä, kun joku yrittää ohittaa auton liian nopeasti.

If the kun-clause comes first, you also separate it:

Kun joku yrittää ohittaa auton liian nopeasti, risteyksessä on paljon jännitystä.

What is the exact function of kun here? Is it the same as English when?

Yes, here kun is a temporal conjunction meaning when / whenever:

kun joku yrittää ohittaa auton liian nopeasti
= when / whenever someone tries to overtake a car too fast

Some nuances:

kun = when / whenever (time), but in colloquial speech it can also mean because.
In this sentence it’s clearly temporal, not causal.
If you want to emphasize “every time that…”, you could say:
- aina kun joku yrittää… = whenever someone tries…

You generally do not use milloin in this type of clause; milloin is more for question words (“when?”):

Milloin siellä on paljon jännitystä? = When is there a lot of tension there?

Why is it joku yrittää ohittaa, not joku yrittää ohittamaan?

The verb yrittää (to try) normally takes the basic infinitive (1st infinitive) of the following verb:

yrittää tehdä = try to do
yrittää päästä = try to get / to reach
yrittää nukkua = try to sleep
yrittää ohittaa = try to overtake

Using the 3rd infinitive illative (-maan / -mään, like ohittamaan) after yrittää is not standard. So:

joku yrittää ohittaa auton = correct
joku yrittää ohittamaan auton = wrong in standard Finnish

Why is it joku yrittää and not jotain yrittää or jotakuta yrittää?

Joku means someone / some person and here it is the subject of the verb yrittää. Subjects in Finnish normally appear in the nominative case:

kuka? → joku (someone)
joku yrittää = someone tries

Other forms of the same pronoun:

jonkun = of someone (genitive)
jotakuta = someone (partitive, used mostly as object or after some prepositions / postpositions)

Jotain is usually partitive of jokin (“something”) and is not used as a subject meaning “someone”:

jotain hyvää = something good

So for someone tries, you need joku yrittää, not jotain or jotakuta.

Why is it auton and not autoa after ohittaa?

Auton is the genitive singular of auto. In object position, Finnish distinguishes between:

Total object (genitive or nominative) → the action is directed at a complete whole
Partitive object → incomplete, ongoing, repeated, or “some (of) something”

The verb ohittaa (to pass, to overtake) normally takes a total object, because the natural meaning is to overtake the whole car:

ohittaa auton = (fully) overtake the car
ohitin auton = I passed the car

Partitive autoa would be odd here, because “partially overtaking a car” is not a natural concept. You do the action with respect to the whole vehicle.

Note: even though yrittää only means “try” and does not guarantee success, the object case still follows the logic of the embedded verb ohittaa, so auton is correct:

yrittää ohittaa auton = try to overtake the car

Also, in negative sentences, many verbs (including ohittaa) will use the partitive:

Hän ei yritä ohittaa autoa. = He/she doesn’t try to overtake the car.

What is the difference between ohittaa auton and expressions like ajaa auton ohi?

Both can express overtaking, but they differ slightly:

ohittaa auton
- Single verb ohittaa = to pass, to overtake
- Very common in traffic contexts
- Takes a direct object: auton
ajaa auton ohi
- Literally: “to drive past the car”
- A bit more descriptive / periphrastic, but natural

Unnatural or wrong would be:

ajaa ohi autosta – autosta (from the car) is elative and does not fit here.

In traffic language, ohittaa auton is the standard, compact way to say overtake a car.

Why is it liian nopeasti and not liian nopea?

It depends on what you are qualifying:

nopea = fast (adjective, describing nouns)
nopeasti = fast / quickly (adverb, describing verbs)

In this sentence, liian nopeasti modifies the verb phrase (yrittää ohittaa), so you need an adverb:

yrittää ohittaa liian nopeasti
= try to overtake too fast / too quickly

If you wanted to describe a noun, you would use the adjective:

liian nopea auto = a car that is too fast
liian nopea ohitus = an overtaking manoeuvre that is too fast

So:

Verb → liian nopeasti
Noun → liian nopea

Does the present tense on / yrittää mean this happens right now, or generally?

In Finnish, the present tense is often used for general truths, typical situations, and repeated events, just like English “when someone tries to overtake a car, there is a lot of tension”.

Here on and yrittää express a general / habitual situation:

Risteyksessä on paljon jännitystä, kun joku yrittää ohittaa auton liian nopeasti.
= There is a lot of tension at the intersection whenever someone tries to overtake a car too fast.

If you wanted to talk about one specific past event, you’d use the past tense:

Risteyksessä oli paljon jännitystä, kun joku yritti ohittaa auton liian nopeasti.
= There was a lot of tension at the intersection when someone tried to overtake a car too fast.