Learn Hardcore Finnish: Yksi lelu putosi portaisiin, joten nostin sen heti ylös. - One toy fell into the stairs, so I immediately picked it up.

Yksi lelu literally means “one toy”, but very often it works like “one (of the toys)” / “a toy” / “one particular toy”.
If you said just Lelu putosi portaisiin, it would more likely be understood as:
- “The toy fell onto the stairs” – a toy already known from the context.

So:

Yksi lelu putosi portaisiin…
→ One of the toys / a toy fell onto the stairs… (introducing a new toy)
Lelu putosi portaisiin…
→ The toy fell onto the stairs… (referring to a specific, previously mentioned toy)

You can drop yksi if the context already makes it clear which toy you mean, but when introducing a toy for the first time, yksi is very natural.

What verb is putosi, and why does it look so different from the dictionary form pudota?

The dictionary form is pudota = to fall (intransitive: something falls by itself).

Putosi is:

3rd person singular past tense of pudota

Conjugation (just for “it”):

se putoaa = it falls (present)
se putosi = it fell (past)

The stem for most forms is puto- (the d disappears in many forms, which is a regular pattern for this verb type):

infinitive: pudota
present: putoan, putoat, putoaa…
past: putosin, putosit, putosi…

So the sentence’s first clause literally is:

Yksi lelu putosi portaisiin = One toy fell onto/into the stairs.

Also note the contrast:

pudota – intransitive: Lelu putosi = The toy fell.
pudottaa – transitive: Pudotin lelun portaisiin = I dropped the toy onto the stairs.

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

Portaisiin is the illative plural of porras (a step / stair).

Basic forms:

singular nominative: porras = (one) step / stair
plural nominative: portaat = stairs
illative plural: portaisiin = into the stairs / onto the stairs

The illative case answers “mihin?” = “to where / into where?” and is used for movement towards/into a place.

So:

portaat – the stairs (as a thing)
portaisiin – (to) the stairs, into/on(to) the stairs

That’s why we use portaisiin with putosi:

Yksi lelu putosi portaisiin.
Literally: One toy fell into the stairs.
Naturally in English: One toy fell onto the stairs.

Could you also say portaille instead of portaisiin? Is there a difference?

Yes, you can also say portaille:

portaille is the allative plural of porras.

Both are used with stairs:

illative (portaisiin) – “into/to the stairs” (into the stair area)
allative (portaille) – “onto the stairs / onto the steps” (onto their surface)

In this context:

Yksi lelu putosi portaisiin – the toy ended up down among / on the stairs.
Yksi lelu putosi portaille – the toy fell onto the steps.

Both are grammatically correct and would usually be understood the same in everyday speech. The nuance is small; learners don’t need to worry too much about the difference here.

What exactly does joten mean, and how is it different from koska or siksi?

joten is a conjunction meaning “so / therefore”.

It introduces a result of what was said before:

Yksi lelu putosi portaisiin, joten nostin sen heti ylös.
→ One toy fell onto the stairs, so I immediately picked it up.

Compare:

koska = because (introduces a reason)
- Myöhästyin, koska bussi oli myöhässä.
  = I was late because the bus was late.
joten = so, therefore (introduces a consequence)
- Bussi oli myöhässä, joten myöhästyin.
  = The bus was late, so I was late.
siksi = for that reason / therefore (an adverb, not a conjunction)
- Bussi oli myöhässä, ja siksi myöhästyin.
  = The bus was late, and for that reason I was late.

In your sentence, we are expressing a consequence of the toy falling, so joten is the natural choice.

Why is there a comma before joten?

In Finnish punctuation, you normally put a comma between two full clauses (each with its own finite verb), even when English might not.

Here we have two main clauses:

Yksi lelu putosi portaisiin – One toy fell onto the stairs.
nostin sen heti ylös – I immediately lifted it up.

They are linked with joten (so, therefore), so Finnish writes:

Yksi lelu putosi portaisiin, joten nostin sen heti ylös.

Rule of thumb:

Put a comma before joten, mutta, vaikka, koska, että, etc. when they start a new clause.
With ja and tai, the comma rules are a bit looser, but joten almost always has a comma before it.

In nostin sen heti ylös, why is the pronoun sen used, and not se or sitä?

The pronoun se (“it”) has different case forms:

se – nominative (usually subject)
sen – genitive/accusative (often total object)
sitä – partitive (often partial/ongoing object)

Here, the toy is the object of nostin (I lifted), and it is lifted completely, as a whole, in a single completed action.

That calls for a total object, which is:

sen (not sitä) in this kind of past-tense affirmative sentence.

So:

Nostin sen heti ylös.
= I lifted it up right away. (finished, whole object)

Other contrasts:

Se putosi. – It fell. (subject, nominative)
En nostanut sitä. – I didn’t lift it (up). (negative → partitive object)
Nostin sitä hitaasti. – I was (in the process of) lifting it slowly. (ongoing / incomplete → partitive)

So sen is exactly right here because the toy is a completed, definite object in an affirmative clause.

Is ylös necessary here? Could we just say nostin sen heti?

You could say Nostin sen heti, and it would be grammatically correct and understandable:

nostaa already means to lift / to raise, which usually implies “up”.

However, ylös (up(wards)) adds explicit direction and a bit of emphasis:

nostin sen heti ylös = I immediately lifted it up (from below).

Reasons Finns often include ylös here:

It makes the physical movement clearer: from the stairs up into your hand or off the floor.
It contrasts nicely with other directions, like alas (down), ulos (out), etc.

So:

Nostin sen heti. – perfectly fine: I lifted it immediately.
Nostin sen heti ylös. – slightly more vivid: I lifted it right up (off the stairs).

Why is there no word for “I” in nostin?

Finnish verbs mark the subject in the verb ending, so the subject pronoun is often omitted.

nostin = I lifted
- -n ending = 1st person singular
nostit = you lifted (singular)
nosti = he/she/it lifted

Because the verb already clearly shows the subject, minä (I) is usually left out unless you want to emphasise it:

Nostin sen heti ylös. – I lifted it up immediately. (neutral)
Minä nostin sen heti ylös. – I (as opposed to someone else) lifted it up immediately. (emphasising I)

Can the word order in nostin sen heti ylös be changed, and does it change the meaning?

Finnish word order is fairly flexible, and different orders mostly affect emphasis, not basic meaning.

All of these are grammatically possible:

nostin sen heti ylös – neutral, very natural:
- verb + short object pronoun + adverb + direction
nostin heti sen ylös – also possible:
- slight emphasis on heti (how quickly it was done)
sen nostin heti ylös – emphasises sen (“that one in particular”):
- That one I immediately lifted up.
heti nostin sen ylös – strong focus on heti:
- Right away I lifted it up.

In everyday speech, nostin sen heti ylös and nostin heti sen ylös are probably the most natural here. The others are more about highlighting a contrast or adding stylistic emphasis.