Lentoyhtiö lähettää viestin, jos matkatavara jää väärään kaupunkiin.

Lentoyhtiö is a compound noun:

• lento = flight
• yhtiö = company

So lentoyhtiö literally means flight company, i.e. airline.

Finnish uses compound words very freely, much more than English does, so long nouns like this are very common.

Because viestin is the object in this sentence, and Finnish marks objects with case.

Here, lähettää viestin means sends a/the message as a complete action. For a singular total object, Finnish often uses a form that looks like the genitive:

• viesti = message
• viestin = the message / a message, as a completed object here

So:

• Lentoyhtiö lähettää viestin = The airline sends a message

This is one of the common object patterns in Finnish.

Yes, but Finnish often treats matkatavara as a collective singular noun, much like English luggage or baggage.

• matka = trip, travel
• tavara = thing, goods, stuff
• matkatavara = luggage, baggage

So even if there are several bags, Finnish can still say matkatavara in the singular.

If you wanted to talk about separate individual items, you might use another expression, but here the singular is perfectly natural.

Jää is the 3rd person singular of jäädä, which can mean:

• to stay
• to remain
• to be left
• to get left behind

In this sentence, it means something like:

• if the luggage is left in the wrong city
• if the luggage ends up in the wrong city
• if the luggage remains in the wrong city

So it is not just a neutral stay. In travel situations, jäädä often suggests that something did not go where it was supposed to go.

Väärään kaupunkiin is in the illative case, which often means into or to a place.

• väärä = wrong
• kaupunki = city
• kaupunkiin = into/to the city
• väärään kaupunkiin = into/to the wrong city

The adjective must agree with the noun, so both change:

• väärä → väärään
• kaupunki → kaupunkiin

Even though English says in the wrong city, Finnish uses the illative here because the idea is that the luggage has ended up there or been left there.

That would also be grammatically possible in some contexts, but it would shift the nuance.

• väärässä kaupungissa = in the wrong city
  This focuses more on location.
• väärään kaupunkiin = to/into the wrong city
  This focuses more on the result or destination: the luggage ended up there.

With luggage and travel, jää väärään kaupunkiin sounds natural because the bag has failed to continue to the correct destination and has been left behind in the wrong place.

Finnish often uses the present tense for future meaning when the future is understood from context.

So:

• Lentoyhtiö lähettää viestin = The airline sends / will send a message
• jos matkatavara jää väärään kaupunkiin = if the luggage is left / ends up in the wrong city

This is completely normal Finnish. There is no separate future tense in everyday Finnish like English will send.

Jos means if and introduces a condition.

The sentence structure is:

• main clause: Lentoyhtiö lähettää viestin
• conditional clause: jos matkatavara jää väärään kaupunkiin

So the whole sentence means:

• The airline sends a message if the luggage is left in the wrong city.

Unlike English, Finnish does not need a word like then here.

Also, Finnish word order after jos stays fairly normal:

• jos matkatavara jää väärään kaupunkiin not something like English-style verb movement.

Yes. Both can relate to a message, but viesti is the normal modern everyday word, especially for:

• text messages
• notifications
• messages from a company or system

So lähettää viestin is exactly what you would expect for an airline sending a notification.

Sanoma exists, but it is less neutral in many contexts and can sound more literary, formal, or specialized.

Finnish does not have articles like English a/an and the.

So:

• lentoyhtiö can mean an airline or the airline
• viestin can mean a message or the message
• matkatavara can mean luggage / the luggage

The exact meaning depends on context. In this kind of sentence, English usually chooses the airline and a message or the airline sends a message.

Yes, Finnish word order is fairly flexible, although the original sentence is the most neutral.

Neutral version:

• Lentoyhtiö lähettää viestin, jos matkatavara jää väärään kaupunkiin.

You could also say:

• Jos matkatavara jää väärään kaupunkiin, lentoyhtiö lähettää viestin.

That puts the if clause first, similar to English If the luggage is left in the wrong city, the airline sends a message.

Both are natural. The difference is mostly about emphasis and flow, not basic meaning.

Yes. Lähettää is the normal verb for to send, whether the thing being sent is:

• a message
• an email
• a text
• a package
• information

So lähettää viestin can mean sending a message in many ways, including an automatic text or app notification from an airline.

There are two clauses, and each has its own subject.

1. Lentoyhtiö lähettää viestin

   • subject: lentoyhtiö = the airline
   • verb: lähettää = sends

2. jos matkatavara jää väärään kaupunkiin

   • subject: matkatavara = the luggage
   • verb: jää = is left / remains / ends up

So the luggage is not doing the sending. The airline sends the message, and the luggage is what gets left behind.

Yes, that is a very good observation.

Grammatically, it is not a Finnish passive. The subject is matkatavara, and the verb is active: jää.

But semantically, the luggage is not acting intentionally. English often expresses this with a passive-like idea:

• the luggage is left in the wrong city

Finnish often uses jäädä for exactly this kind of situation, where something ends up somewhere unintentionally, without needing a passive structure.

A few natural translations are:

• The airline sends a message if the luggage is left in the wrong city.
• The airline sends a message if the luggage ends up in the wrong city.
• The airline will send a message if the luggage is left in the wrong city.

All of these fit. The best choice depends on the context and the style of English you want.