Huomenna minulla on aikainen lento, joten laitan käsimatkatavaran valmiiksi jo illalla.

Finnish usually expresses possession with the verb olla (to be) plus the possessor in the adessive case.

So:

• minulla = on/at me
• on = is

Literally, minulla on aikainen lento is something like At me is an early flight, but in natural English it means I have an early flight.

This is the normal Finnish way to say that someone has something:

• Minulla on auto. = I have a car.
• Hänellä on aikaa. = He/She has time.

In the possession structure minulla on ..., the thing possessed is normally in the nominative.

So here:

• aikainen = early
• lento = flight

Together, aikainen lento is the thing the speaker has. That is why it stays in the basic form here.

Also, the adjective agrees with the noun:

• aikainen lento = nominative singular
• if the noun changed case, the adjective would usually change too

Joten means so, therefore, or thus. It introduces a result/consequence.

In this sentence:

• Huomenna minulla on aikainen lento, joten...
• Tomorrow I have an early flight, so...

By contrast, koska means because and introduces a reason.

Compare:

• Minulla on aikainen lento, joten laitan käsimatkatavaran valmiiksi.
  = I have an early flight, so I’ll get the carry-on ready.

• Laitan käsimatkatavaran valmiiksi, koska minulla on aikainen lento.
  = I’ll get the carry-on ready because I have an early flight.

So joten points forward to the consequence, while koska points to the reason.

Laitan is the 1st person singular present tense of laittaa.

So:

• laittaa = to put / to place / to make ready / to set
• laitan = I put / I will put

In Finnish, the present tense often also covers a near future meaning when the context makes it clear. Since the sentence is about tomorrow and tonight, laitan is naturally understood as I’ll put / I’ll get ready / I’ll pack.

Here käsimatkatavaran is the object of laitan, and the -n marks a total object in the singular.

That usually means the action is seen as completed or directed at the whole item:

• laitan käsimatkatavaran valmiiksi = I’ll get the carry-on ready / I’ll pack the carry-on completely

The basic dictionary form is käsimatkatavara. In this sentence it becomes käsimatkatavaran because of its object role.

A useful contrast:

• Luen kirjan. = I’ll read the book / I read the whole book
• Luen kirjaa. = I’m reading a book / I’m reading the book (ongoing, incomplete)

Finnish often leaves possession unstated when it is obvious from context.

So even though the English meaning may say my carry-on, Finnish can simply say:

• laitan käsimatkatavaran valmiiksi

Because the speaker is talking about their own trip, it is naturally understood as my carry-on.

If you wanted to make it explicit, you could say something like:

• laitan käsimatkatavarani valmiiksi

But in everyday Finnish, that extra marking is often unnecessary.

Yes, it is a compound word, which is very common in Finnish.

It is made up of:

• käsi = hand
• matka = trip/journey
• tavara = thing/goods/luggage

So käsimatkatavara literally suggests something like hand-travel-goods, meaning carry-on luggage / hand luggage.

Finnish forms a lot of vocabulary this way, by joining nouns into one long word.

Valmiiksi means ready in the sense of into a ready state or so that it is ready.

It comes from valmis = ready, and the ending -ksi is the translative case. The translative often expresses a change into a state.

So:

• valmis = ready
• valmiiksi = into ready / ready for use

That is why laitan käsimatkatavaran valmiiksi means not just I put the carry-on, but I get the carry-on ready.

A similar pattern:

• maalata seinä punaiseksi = paint the wall red
• tehdä asia helpoksi = make the thing easy

Jo usually means already.

Here it adds the idea of earlier than one might expect:

• jo illalla = already in the evening

So the speaker is saying that because the flight is early tomorrow, they will prepare the carry-on as early as the evening before.

Without jo, the sentence would still make sense, but jo gives a clearer feeling of planning ahead.

Illalla is the adessive form of ilta (evening).

With parts of the day, Finnish often uses the adessive to mean during / in / on that time period:

• aamulla = in the morning
• päivällä = during the day
• illalla = in the evening
• yöllä = at night

So jo illalla means already in the evening.

This is a very common time expression pattern in Finnish.

Yes. Finnish word order is fairly flexible, and changing it often changes the focus or emphasis, not the basic meaning.

The original sentence begins with Huomenna to set the time frame right away:

• Huomenna minulla on aikainen lento...

You could also say:

• Minulla on huomenna aikainen lento...

That is also natural. It simply puts the focus a bit differently.

In general, Finnish often moves time words like huomenna toward the beginning when they are the topic or setting of the sentence.

Yes, huomenna means tomorrow.

It is an adverbial time expression, and learners often just need to memorize it as the normal word for tomorrow. It does not behave like a regular noun phrase in this sentence.

Compare:

• tänään = today
• huomenna = tomorrow
• eilen = yesterday

These are very common fixed time words in Finnish and do not need to be analyzed like ordinary noun case forms when you are starting out.