Jos lento peruuntuisi, varaisin hotellihuoneen ja lähettäisin pomolle viestin heti.

That -isi- is the marker of the conditional mood in Finnish.

In this sentence, the conditional means something like would in English:

• peruuntuisi = would be canceled
• varaisin = I would book
• lähettäisin = I would send

A rough breakdown:

• varata → varaisin
• lähettää → lähettäisin
• peruuntua → peruuntuisi

In varaisin and lähettäisin, the final -n shows 1st person singular: I would.

That is a very common question. Finnish and English handle hypothetical if-sentences differently.

In Finnish, when the situation is hypothetical / unlikely / imagined, it is normal to use the conditional in both clauses:

• Jos lento peruuntuisi, varaisin hotellihuoneen.

Literally, this looks like:

• If the flight would be canceled, I would book a hotel room.

But that is just how Finnish expresses this type of unreal or hypothetical condition.

Compare:

• Jos lento peruuntuu, varaan hotellihuoneen.
  = If the flight gets canceled, I'll book a hotel room.
  This sounds more like a real possibility.

• Jos lento peruuntuisi, varaisin hotellihuoneen.
  = If the flight were canceled, I would book a hotel room.
  This is more hypothetical.

So the Finnish sentence is perfectly normal.

The difference is:

• perua = to cancel something
• peruuntua = to be canceled / get canceled

So:

• Yhtiö peruu lennon.
  = The company cancels the flight.

• Lento peruuntuu.
  = The flight is canceled / gets canceled.

In your sentence, lento is the thing that gets canceled, so Finnish uses peruuntua, not perua.

That is why:

• Jos lento peruuntuisi...
  = If the flight were canceled...

Finnish often leaves out subject pronouns when the verb ending already tells you who the subject is.

Here:

• varaisin = I would book
• lähettäisin = I would send

The ending -n shows 1st person singular, so minä is not necessary.

You could say:

• Jos lento peruuntuisi, minä varaisin hotellihuoneen...

But that would usually add emphasis, like I would book it.

The version without minä is more neutral and natural.

Because hotellihuoneen is the object form here.

In Finnish, the object often changes form depending on whether the action is seen as:

• complete / whole / bounded → often total object
• incomplete / ongoing / part of something → often partitive object

Here, varaisin hotellihuoneen means booking a whole room, a complete action, so Finnish uses the total object form:

• hotellihuoneen

This form looks like the genitive singular.

So:

• varaisin hotellihuoneen = I would book a hotel room

If you only memorize one practical point here, memorize this: after many affirmative verbs, a single whole object often appears in this -n form.

For the same reason as hotellihuoneen: it is the object of the verb, and here it is a complete, whole message.

• lähettäisin viestin = I would send a message

So viestin is the object form used for a single completed item.

Compare the basic dictionary form:

• viesti = message

But in the sentence:

• lähettäisin pomolle viestin
  = I would send the boss a message

The ending -lle is the allative case, which often means:

• to
• onto
• for

Here it means to:

• pomolle = to the boss

So:

• lähettäisin pomolle viestin
  = I would send a message to the boss

This is very common in Finnish:

• annan ystävälle kirjan = I give a book to a friend
• soitan äidille = I call my mother

So pomolle is simply to the boss.

Good catch. Pomolle by itself literally means to the boss. It does not explicitly say my.

A more explicit version would be:

• pomolleni = to my boss
• minun pomolleni = to my boss (more explicit)

So why might a translation say my boss?

Because in context, if I say I would send the boss a message, it is often naturally understood as my boss. Finnish often leaves possession unspoken when it is obvious from the situation.

Still, if you want to be fully precise in standard Finnish, my boss is better as:

• lähettäisin pomolleni viestin heti

So the original sentence is natural, but slightly less explicit about possession.

Heti means:

• immediately
• right away

It is an adverb.

So:

• lähettäisin pomolle viestin heti
  = I would send my boss a message immediately

Putting heti at the end is very natural in Finnish. It modifies the action and gives the sentence a smooth neutral flow.

You could move it for emphasis:

• heti lähettäisin...
• lähettäisin heti pomolle viestin

But the original order is completely normal.

Finnish word order is more flexible than English word order, because endings carry a lot of the grammatical information.

The original sentence has a very neutral order:

• Jos lento peruuntuisi, varaisin hotellihuoneen ja lähettäisin pomolle viestin heti.

But other versions are possible, for example:

• Jos lento peruuntuisi, lähettäisin heti pomolle viestin ja varaisin hotellihuoneen.
• Hotellihuoneen varaisin, jos lento peruuntuisi.
  This sounds more marked or emphatic.

Even though word order can move around, the original is probably the best model for a learner because it is clear and natural.

Because Jos lento peruuntuisi is a subordinate clause:

• If the flight were canceled

In Finnish, when a subordinate clause comes before the main clause, it is separated by a comma:

• Jos lento peruuntuisi, varaisin hotellihuoneen...

This is standard Finnish punctuation.

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

Instead, Finnish uses the present tense or the conditional, depending on the meaning.

Here, the sentence refers to a possible future situation, but it is presented as hypothetical, so Finnish uses the conditional:

• Jos lento peruuntuisi...
• varaisin... lähettäisin...

If it were a more straightforward real future possibility, Finnish would often use present-tense forms:

• Jos lento peruuntuu, varaan hotellihuoneen ja lähetän pomolle viestin heti.

So Finnish expresses future meaning through context, not a special future verb form.

Their dictionary forms are:

• varata = to book / reserve
• lähettää = to send

In the conditional 1st person singular, they become:

• varaisin = I would book
• lähettäisin = I would send

A useful learner-level way to think about it is:

• take the verb stem
• add the conditional marker -isi-
• add the personal ending

So:

• vara- + isi + n → varaisin
• lähettä- + isi + n → lähettäisin

You do not need to master every stem rule immediately, but it is useful to recognize that -isin often means I would.

For example:

• menisin = I would go
• ostaisin = I would buy
• sanoisin = I would say