Toisen lokeron ovi aukesi vasta, kun kirjoitin noutokoodin uudestaan.

Because Finnish is showing a relationship like the door of the second locker.

• toinen = second / other
• lokero = compartment, locker
• ovi = door

When one noun belongs to or specifies another noun, Finnish often uses the genitive:

• toisen = of the second / of the other
• lokeron = of the locker

So:

• toisen lokeron ovi = the door of the second locker / the other locker’s door

This is a very common Finnish structure:

• talon ovi = the house’s door / the door of the house
• auton avain = the car key

So toisen lokeron ovi is a noun phrase built step by step:

1. lokero → lokeron
2. toinen → toisen
3. toisen lokeron = of the second locker
4. toisen lokeron ovi = the second locker’s door

Because it is in the genitive singular.

The basic form is:

• toinen = second / other

The genitive form is:

• toisen = of the second / of the other

Here it modifies lokeron, which is also in the genitive, because together they form the phrase of the second locker.

This is normal agreement in Finnish:

• toinen lokero = the second locker
• toisen lokeron = of the second locker

So toisen is not random; it matches the case of the noun it describes.

Because lokeron is the genitive singular of lokero.

• lokero = locker / compartment
• lokeron = of the locker

In this sentence, lokeron is part of the phrase:

• toisen lokeron ovi = the second locker’s door

Finnish often uses the genitive before another noun to express possession or close connection.

Compare:

• huoneen ovi = the room’s door
• kaapin ovi = the cupboard door
• toisen lokeron ovi = the other/second locker’s door

Because ovi is the subject of the clause.

The thing that opened was the door, so ovi is the subject:

• Toisen lokeron ovi aukesi = The second locker’s door opened

Subjects are typically in the nominative in Finnish, especially in ordinary affirmative clauses like this one.

So:

• ovi = nominative subject
• aukesi = opened

If you said oven, that would be the genitive/accusative form and would not fit here as the subject of opened.

Yes, here aukesi means opened in the sense of came open or opened by itself.

It is the past tense of aueta:

• aueta = to open, to come open
• aukesi = opened / came open

This verb focuses on the door becoming open, not necessarily on who opened it.

That is different from avata:

• avata = to open something
• avasin oven = I opened the door

But:

• ovi aukesi = the door opened

So in this sentence, Finnish is describing the result:

• The door opened only when...

Yes, in many contexts avautui could also work.

• aukesi comes from aueta
• avautui comes from avautua

Both can mean opened / came open, and in everyday language they are often quite close.

Possible nuance:

• aukesi can sound a bit more direct and simple: opened
• avautui can sometimes sound slightly more neutral or process-focused: became open

In this sentence:

• Toisen lokeron ovi aukesi...
• Toisen lokeron ovi avautui...

Both are understandable and natural. Aukesi is very idiomatic here.

Here vasta means only in the sense of not until.

So:

• ovi aukesi vasta, kun...
  = the door only opened when... = the door did not open until...

This is a very common use of vasta.

Examples:

• Hän tuli vasta illalla. = He only came in the evening.
• Ymmärsin vasta myöhemmin. = I only understood later.
• Ovi aukesi vasta, kun kirjoitin koodin uudestaan. = The door didn’t open until I entered the code again.

So vasta marks that something happened later than expected, or only after a certain point.

Here kun means when.

• vasta, kun kirjoitin... = only when I wrote/entered...

Finnish kun most often means when, introducing a time clause.

In some contexts, especially spoken language, kun can also feel close to because, but that is not what is happening here.

In this sentence it clearly introduces the event that triggered the opening:

• first, I re-entered the pickup code
• then, the door opened

So the meaning is temporal:

• when I entered the pickup code again

Because the whole sentence describes a past event.

• aukesi = opened
• kirjoitin = I wrote / I typed / I entered

Finnish often keeps both verbs in the past when both actions happened in the past:

• Toisen lokeron ovi aukesi vasta, kun kirjoitin noutokoodin uudestaan.
• The door only opened when I entered the pickup code again.

The sequence is:

1. I entered the code again
2. the door opened

Since both actions are part of a past situation, kirjoitin is in the imperfect past tense.

Because it is the object of kirjoitin, and here it is a total object.

• noutokoodi = pickup code / collection code
• noutokoodin = the pickup code

In Finnish, many completed actions take the object in the genitive singular form (which often looks the same as the so-called accusative in singular nouns).

Here the idea is that the whole code was entered:

• kirjoitin noutokoodin = I entered/wrote the pickup code

Compare the general idea:

• complete whole object → often -n
• partial/incomplete/ongoing → often partitive

So noutokoodin suggests the action was completed: the code was entered in full.

Yes, literally kirjoitin means I wrote, but in Finnish it is also commonly used for typing in or entering text or numbers.

So with a code, kirjoittaa koodi can mean:

• write the code
• type in the code
• enter the code

In English, enter the code is often the most natural translation, but Finnish may still use kirjoittaa.

So:

• kirjoitin noutokoodin uudestaan
  = I entered the pickup code again

Yes, uudestaan means again.

More literally, it is something like anew or from the beginning again, but in normal usage it often simply means again.

So:

• kirjoitin noutokoodin uudestaan = I entered the pickup code again

A close alternative is uudelleen:

• kirjoitin noutokoodin uudelleen

Both are common. In many situations they are interchangeable.

A slight tendency:

• uudestaan can sound a bit more conversational
• uudelleen can sound a bit more neutral or formal

The word order is natural Finnish.

• Toisen lokeron ovi = subject
• aukesi = verb
• vasta = only / not until
• kun... = when...

So:

• Toisen lokeron ovi aukesi vasta, kun...

This puts the main statement first:

• the door opened only when...

If you moved words around, the emphasis could change a little, but the current version is very normal and clear.

Finnish word order is more flexible than English, but it is not random. Here the sentence flows naturally from:

1. what we are talking about (the door)
2. what happened (opened)
3. the limiting condition (only when...)

It can potentially mean either, depending on context.

• toinen can mean second
• it can also mean the other

So toisen lokeron ovi could mean:

• the second locker’s door
• the other locker’s door

Usually the wider context tells you which one is meant.

For example:

• if there are numbered compartments, toisen may mean the second
• if there are two lockers being compared, it may mean the other

Finnish often leaves this kind of distinction to context, where English might force a more specific choice.

Yes. vasta, kun... is a very useful pattern meaning only when... or not until...

Structure:

• [main event] vasta, kun [triggering event]

In this sentence:

• Ovi aukesi vasta, kun kirjoitin noutokoodin uudestaan.
• The door didn’t open until I entered the pickup code again.

More examples:

• Ymmärsin vasta, kun hän selitti sen uudestaan.
  = I only understood when he explained it again.
• Bussi tuli vasta, kun oli jo pimeää.
  = The bus only came when it was already dark.

So this is a very common and useful construction to remember.