Saatuani ovikoodin lähetin sen myös siskolleni, jotta hän pääsee sisään, jos olen myöhässä.

Saatuani means after I had received / once I received.

It comes from the verb saada (to get, receive) and is a compact Finnish way to express after doing something.

So:

• Saatuani ovikoodin = after receiving the door code
• more literally: having received the door code, I...

This structure is very common in written and fairly formal Finnish.

It is built from the verb saada using the so-called temporal construction.

Very roughly:

• saatua → participle stem related to saatu- (received / having received)
• plus -a-ni type ending showing when I had...

In practice, you can learn saatuani as a fixed pattern meaning:

• after I received
• once I had received

Similar examples:

• Tultuani kotiin... = After I came home...
• Syötyäni... = After I ate / having eaten...

The -ni shows the person: I.

In this construction, the ending tells whose action it is:

• saatuani = after I received
• saatuasi = after you received
• saatuaan = after he/she received

So the subject of Saatuani is the same person as the subject of the main verb lähetin (I sent).

That is an important rule here: in this kind of structure, the understood subject is normally the same as in the main clause.

Because ovikoodin is the object form used here.

The basic form is:

• ovikoodi = door code

In the sentence:

• Saatuani ovikoodin = after receiving the door code

the code is the thing received, so it appears in the object form ovikoodin.

For learners, it is often easiest to think of this as the form Finnish commonly uses for a complete, specific object in sentences like this.

Sen means it and refers back to ovikoodin (the door code).

So:

• lähetin sen = I sent it

Finnish often uses se / sen for things, and in everyday language even for people sometimes, though here it clearly refers to the code.

Siskolleni means to my sister.

It breaks down like this:

• sisko = sister
• siskolle = to the sister / to a sister
• siskolleni = to my sister

The ending -lle is the allative case, which often expresses movement or transfer to someone.

So:

• lähetin sen siskolleni = I sent it to my sister

The -ni at the end means my.

Myös means also / too.

Here:

• lähetin sen myös siskolleni = I sent it also to my sister

In natural English we would usually say:

• I also sent it to my sister
• or I sent it to my sister too

In Finnish, myös often goes directly before the part it emphasizes. Here it emphasizes siskolleni: not only to someone else, but also to my sister.

Jotta means so that / in order that.

It introduces a purpose clause:

• jotta hän pääsee sisään = so that she can get in

So the speaker sent the code to the sister for a purpose: so she will be able to enter.

Compared with some other conjunctions:

• että often means that
• jotta specifically expresses purpose

Because this part describes the intended result or future possibility from the point of view of the situation.

Main clause:

• lähetin = I sent (past)

Purpose clause:

• jotta hän pääsee sisään = so that she can get in

Even though the sending happened in the past, the sister’s getting in is something that would happen later, when needed. Finnish often uses the present tense in such clauses where English might also use present:

• I sent it so that she can get in

This is normal Finnish.

Sisään means inside / in / into the building, depending on context.

With päästä (to get, to gain access, to manage to go), the phrase:

• päästä sisään = to get in / to get inside

So:

• hän pääsee sisään = she can get in

Here it clearly means getting into the building or through the door.

Finnish does not have a separate future tense like English does.

So:

• jos olen myöhässä literally = if I am late
• natural English here = if I’m late / if I end up being late

Even when English talks about the future, Finnish usually uses the present tense:

• jos olen myöhässä = if I’m late
• kun hän tulee = when he/she comes

That is completely normal.

Myöhässä means late.

It is actually based on an inessive form, but for most learners it is best to learn it as a set expression:

• olla myöhässä = to be late

Examples:

• Olen myöhässä. = I am late.
• Hän on myöhässä töistä. = He/She is late from work or late coming from work, depending on context.

So in your sentence:

• jos olen myöhässä = if I’m late

It is not stated separately, because it is understood to be the same as the subject of the main clause.

Main clause:

• lähetin = I sent

So Saatuani must also refer to I:

• Saatuani ovikoodin, lähetin...
• After I received the door code, I sent...

This is a key feature of this construction: the person in the -tuani / -tyäni type form is normally the same as the main clause subject.

Yes. A more straightforward version would be:

• Kun sain ovikoodin, lähetin sen myös siskolleni, jotta hän pääsee sisään, jos olen myöhässä.

This means the same thing:

• When/after I got the door code, I also sent it to my sister so that she can get in if I’m late.

The original version with Saatuani is a bit more compact and more formal or literary in style. The version with kun sain is often easier for learners to understand.