Learn Hardcore Finnish: Rohkea opettaja käytti liikennevaloja leikkinä, jotta lapset oppivat, milloin saa ylittää kadun. - A brave teacher used traffic lights as a game so that the children learn when you are allowed to cross the street.

You could also say opettaja oli rohkea (“the teacher was brave”), but that would be a separate sentence. Here it’s just a normal adjective–noun phrase functioning as the subject of the sentence.

How do I know that käytti is past tense and what person is it?

Käytti comes from the verb käyttää (“to use”).

The stem is käyttä-
The personal ending -i here marks past tense in the 3rd person singular.

So:

hän käyttää – he/she uses
hän käytti – he/she used

In this sentence, rohkea opettaja is the subject, so:

rohkea opettaja käytti = “the brave teacher used.”

Because opettaja is 3rd person singular, the verb form is also 3rd person singular (no separate pronoun needed in Finnish).

Why is liikennevaloja in that form, and what case is it?

Liikennevaloja is the partitive plural of liikennevalo (“traffic light”; literally traffic + light).

Singular nominative: liikennevalo – a traffic light
Plural nominative: liikennevalot – traffic lights
Plural partitive: liikennevaloja – (some) traffic lights

The verb käyttää (“to use”) often takes its object in the partitive when:

The amount is not specified or is “some of”:
- Käytin kyniä. – I used (some) pens.
The focus is not on a single, clearly delimited whole object, but on using something as a material/tool/means.

Here, liikennevaloja suggests “used (some) traffic lights” rather than a specific fixed set of lights as a single “whole.” It sounds natural because the focus is that she used traffic lights as a prop in a game, not that she used a particular counted number of them.

What does leikkinä mean, and why does it end in -nä?

Leikkinä is the essive case of leikki (“game, play”).

Base form (nominative): leikki – a game
Essive: leikkinä – as a game

The essive -na/-nä ending often means “as X, in the role of X, in the form/state of X.” For example:

opettajana – as a teacher
esimerkkinä – as an example
leikkinä – as a game

So käytti liikennevaloja leikkinä literally means “used traffic lights as a game” (i.e., turned them into some kind of game or playful activity).

Could you also say leikiksi instead of leikkinä? What’s the difference?

Yes, leikiksi is also possible, but the nuance is different.

leikkinä = essive “as a game”
leikiksi = translative “into a game”

Essive (leikkinä) focuses on the role/state:

She used the traffic lights in the role of a game, as something playful.

Translative (leikiksi) focuses more on change into something:

She turned the use of traffic lights into a game.

Both can work contextually, but leikkinä is slightly more neutral here: she used them as a game-like activity.

What does jotta mean, and how is it different from että?

Jotta introduces a purpose clause and usually means “so that / in order that.”

jotta lapset oppivat – so that the children learned
jotta lapset oppisivat – so that the children would learn

Että usually means just “that” and introduces a content clause (like reported speech or thoughts):

Tiedän, että lapset oppivat. – I know that the children learn/learned.

So:

Use jotta when expressing purpose or goal.
Use että when you just need “that” for a statement.

In your sentence, jotta shows why the teacher used the traffic lights as a game: for the purpose that the children would learn when it’s allowed to cross the street.

Why is it lapset oppivat and not lapset oppisivat after jotta?

Both are possible, but they have different nuances:

jotta lapset oppisivat – “so that the children would learn.”
- Uses the conditional (oppisivat) and emphasizes the intended goal, whether or not it actually happened.
jotta lapset oppivat – “so that the children learned.”
- Uses past tense indicative (oppivat) and in context often suggests the goal is presented as being actually achieved in the story.

Many textbooks first teach jotta + conditional for purpose (jotta + oppisivat). The version with oppivat sounds more like the narrator is describing a successful, actual outcome. Stylistically, some speakers might still prefer oppisivat here, but oppivat is understandable and can sound like a narrative description of what happened.

What exactly does oppivat mean, and how is it formed?

Oppivat is the 3rd person plural past tense of oppia (“to learn”).

Present tense:

he oppivat (colloquially often he oppii, but standard is he oppivat) – they learn

Past tense:

he oppivat – they learned

So in this sentence, lapset oppivat = “the children learned.”

Formally, both the present and past 3rd person plural of this verb have the same written form (oppivat); you understand the tense from context and from the rest of the sentence. In this case, because the main verb is käytti (clearly past), oppivat is naturally interpreted as past as well.

What does milloin saa ylittää kadun literally mean, and why is saa singular?

Milloin saa ylittää kadun literally is:

milloin – when
saa – is allowed / may
ylittää – to cross
kadun – the street (object)

So: “when one is allowed to cross the street” or “when it’s allowed to cross the street.”

Saa is 3rd person singular, but there is no expressed subject like “he” or “they.” This is a common impersonal structure in Finnish:

Täällä saa tupakoida. – One is allowed to smoke here / You can smoke here.
Tänne ei saa tulla. – One must not come here / You can’t come here.

So milloin saa ylittää kadun is a neutral, general rule: when a person is allowed to cross, not tied to any particular individual.

Why is it kadun and not katu or kadulla?

Kadun is the genitive singular of katu (“street”):

nominative: katu – street
genitive: kadun – of the street

As an object, the genitive form kadun here functions as a total object (practically the same role as an accusative object in many languages):

ylittää kadun – to cross the (whole) street

If you used just katu in the nominative, it would sound ungrammatical in this function.

Kadulla is the adessive case (“on the street”):

seistä kadulla – to stand on the street

But here the verb ylittää (“to cross”) needs a direct object: you cross something, so that something (the street) appears as kadun.

Why is there a comma before jotta?

Finnish normally puts a comma before a subordinate clause, including clauses introduced by words like:

että, koska, kun, vaikka, jos, jotta …

So you get:

…leikkinä, jotta lapset oppivat… – “…as a game, so that the children learned…”

The comma marks the boundary between the main clause:

Rohkea opettaja käytti liikennevaloja leikkinä
(The brave teacher used traffic lights as a game)

and the purpose clause introduced by jotta:

jotta lapset oppivat, milloin saa ylittää kadun
(so that the children learned when one is allowed to cross the street)

Could I change the word order, like putting jotta at the beginning?

Yes, you can move the purpose clause earlier:

Jotta lapset oppivat, milloin saa ylittää kadun, rohkea opettaja käytti liikennevaloja leikkinä.

This is grammatically correct. The difference is mostly in emphasis and style:

Original: starts by describing what the teacher did, then gives the reason.
Reordered: starts with the purpose, then tells what the teacher did to achieve it.

Finnish allows relatively flexible word order, but main clause vs. subordinate clause structure and the commas stay the same.

What does liikennevalo literally consist of, and how is the plural formed?

Liikennevalo is a compound of:

liikenne – traffic
valo – light

So literally: traffic light.

Forms:

Singular nominative: liikennevalo – a traffic light
Plural nominative: liikennevalot – traffic lights
Plural partitive: liikennevaloja – (some) traffic lights
Singular partitive: liikennevaloa – (some) of a traffic light / one light in partitive sense

In your sentence, the form is liikennevaloja (partitive plural), used as the object of käytti for the reasons explained earlier (indefinite “some traffic lights” used as a tool/prop in the game).