Kad dođu gosti, sklonit ćemo igračke iz hodnika.

Kad means when here. It is the shorter, very common form of kada. In most everyday contexts, kad and kada mean the same thing.

So both of these are fine:

• Kad dođu gosti...
• Kada dođu gosti...

Kad usually sounds a bit more natural and conversational.

This is a very common pattern in Croatian.

After time words like kad (when), Croatian often uses the present tense form even when the meaning is future. English actually does something similar:

• When the guests arrive, we'll...
• not usually When the guests will arrive...

So Kad dođu gosti literally uses a present form, but it means when the guests arrive in the future.

Dođu is the 3rd person plural present form of doći, which means to come or to arrive.

Here:

• doći = to come / to arrive
• dođu = they come / they arrive

Because the subject is gosti (guests), which is plural, the verb is also plural: dođu.

Gosti is the nominative plural of gost (guest).

It is in the nominative because it is the subject of dođu:

• gost = guest
• gosti = guests

So dođu gosti = the guests arrive.

Croatian does not use articles like the and a/an.

So:

• gosti can mean guests or the guests
• igračke can mean toys or the toys
• hodnik / hodnika can mean hallway or the hallway

Context tells you which meaning is intended.

If you want to be more specific, Croatian can use words like:

• ti gosti = those guests
• te igračke = those toys

But there is no regular article system like in English.

It is the future tense.

Croatian future tense is often made with:

• the infinitive
• plus a short form of htjeti (to want), which functions like an auxiliary

So:

• skloniti = to move away / remove / put aside
• ćemo = we will

Together:

• sklonit ćemo = we will remove / we will put away

When the infinitive comes before the future auxiliary, the final -i is normally dropped in writing:

• skloniti + ćemo → sklonit ćemo

This is standard.

You may also see the future expressed with the auxiliary before the infinitive in some contexts:

• Mi ćemo skloniti igračke.

But in your sentence, sklonit ćemo is the normal form.

Here skloniti means something like:

• move out of the way
• remove
• put away
• clear away

So sklonit ćemo igračke iz hodnika means that the toys will be moved out of the hallway so they are no longer there.

It does not necessarily mean permanently removing them; it often just means putting them somewhere more suitable.

Igračke is in the accusative plural because it is the direct object of sklonit ćemo.

The thing being removed is the toys, so that noun takes the accusative.

From the singular/plural forms:

• igračka = toy
• igračke = toys

For many feminine nouns in -a, the nominative plural and accusative plural look the same, so here igračke could look identical in both cases.

Iz means out of or from, and it requires the genitive case.

So:

• hodnik = hallway
• iz hodnika = out of the hallway / from the hallway

That is why hodnik changes to hodnika.

The phrase shows movement away from the hallway, which fits the meaning of skloniti here.

Yes. The first part, Kad dođu gosti, is a subordinate time clause, and when it comes first, it is separated by a comma:

• Kad dođu gosti, sklonit ćemo igračke iz hodnika.

That comma is standard and expected.

Yes. Croatian word order is fairly flexible, and both are natural:

• Kad dođu gosti, sklonit ćemo igračke iz hodnika.
• Kad gosti dođu, sklonit ćemo igračke iz hodnika.

The meaning is the same: When the guests arrive, we'll remove the toys from the hallway.

The version with dođu gosti may sound a bit more flowing in this sentence, but both are correct.