Tek što smo odlučili kampirati uz rijeku, jedan komarac mi je sletio na ruku.

In this sentence, tek što means something like just after, just when, or barely had... when....

So:

Tek što smo odlučili kampirati uz rijeku, jedan komarac mi je sletio na ruku.

has the sense of:

We had just decided to camp by the river when a mosquito landed on my hand/arm.

It does not mean the literal English combination only that here. As a fixed expression, tek što often introduces an event that had only just happened before another one immediately followed.

Smo is the present-tense auxiliary to be used to form the past tense in Croatian.

The past tense here is:

smo odlučili = we decided / we have decided

Croatian often places short unstressed words like smo near the beginning of the clause, usually in the second position. That is why you get:

Tek što smo odlučili...

rather than something more English-like.

Because Croatian past tense uses a participle that agrees with the subject in number and often gender.

Here the subject is we, so the participle is plural:

• odlučio = he decided
• odlučila = she decided
• odlučilo = it decided
• odlučili = they decided / we decided (masculine or mixed group)
• odlučile = they decided / we decided (all feminine group)

So smo odlučili means we decided, with the masculine/mixed plural form. That is the standard default unless the speakers are known to be all female.

Because after odlučiti meaning to decide, Croatian normally uses an infinitive to say what someone decided to do.

So:

• odlučiti kampirati = to decide to camp
• odlučiti otići = to decide to go
• odlučiti ostati = to decide to stay

This works much like English decide to + verb.

Uz is a preposition that takes the accusative case.

The noun rijeka becomes:

• rijeka = nominative
• rijeku = accusative

So:

uz rijeku = by the river / along the river

This is just the form required by the preposition uz.

No. In this sentence, uz rijeku means by the river, next to the river, or along the river.

Even though rijeku is accusative, that does not automatically mean motion to something. With prepositions, the case is often required by the preposition itself.

So here:

• kampirati uz rijeku = to camp by the river

not to camp to the river.

Jedan literally means one, but it can also work like English a/an when introducing something new into the story.

So:

• jedan komarac = a mosquito
• literally also possible as one mosquito

Here it sounds natural because the mosquito is being introduced for the first time. It can also add a slight storytelling flavor, almost like this one mosquito or some mosquito.

Mi is the short dative form of ja:

• ja = I
• mi = to me

In this sentence, mi shows the person affected:

jedan komarac mi je sletio na ruku
= a mosquito landed on my hand/arm

Croatian often uses the dative pronoun where English would simply use my:

• sletio mi je na ruku = literally landed to me on the hand
• natural English: landed on my hand

So mi is very natural here.

Both mi and je are short unstressed words called clitics. Croatian has special rules about where clitics go, and they usually appear near the beginning of the clause, after the first stressed word or phrase.

Here the first phrase is:

jedan komarac

Then the clitics come:

mi je

So the clause becomes:

jedan komarac mi je sletio...

For a learner, the safest thing is to get used to this as a normal Croatian pattern rather than trying to force English word order onto it.

Because sletjeti involves movement onto a surface.

Croatian often uses:

• na + accusative for motion onto
• na + locative for location on

So:

• na ruku = onto the hand/arm
• na ruci = on the hand/arm

Since the mosquito is landing onto the hand, na ruku is correct.

Sletio is the masculine singular past participle of sletjeti = to land.

It agrees with the subject:

jedan komarac = one mosquito

Since komarac is masculine singular, the verb form is also masculine singular:

• sletio = masculine singular
• sletjela = feminine singular
• sletjelo = neuter singular

So the agreement is perfectly regular.

It can be used for both.

Sletjeti means to land, so it can describe:

• an airplane landing
• a bird landing
• an insect landing

In this sentence, it is used very naturally for a mosquito landing on someone’s hand/arm.

The comma separates the opening time clause from the main event.

First part:

Tek što smo odlučili kampirati uz rijeku
= Just after we decided to camp by the river

Second part:

jedan komarac mi je sletio na ruku
= a mosquito landed on my hand/arm

Croatian often uses a comma in this kind of sentence, just as English often does with an introductory clause.

Yes, sometimes that works well in English.

Depending on context, you could translate the sentence as:

• Just after we decided to camp by the river, a mosquito landed on my hand.
• As soon as we decided to camp by the river, a mosquito landed on my hand.
• We had barely decided to camp by the river when a mosquito landed on my hand.

The exact English wording can change, but the Croatian idea is the same: the second event happened almost immediately after the first.