Kad gosti odu, peremo tanjure.

Kad means when. In sentences like this, it introduces a time clause:

• Kad gosti odu = When the guests leave

Depending on context, kad can also feel like once or after, especially when the action in that clause is completed.

Croatian has no articles, so there is no direct equivalent of the or a/an.

So:

• gosti can mean guests or the guests
• tanjure can mean plates or the plates

You understand which one is meant from context.

Gosti is the nominative plural of gost meaning guest.

It is in the nominative because it is the subject of the verb odu:

• gosti odu = the guests leave

A learner might expect another ending, but here gosti is simply the normal plural subject form.

Odu is the 3rd person plural present form of the verb otići, which means to go away / to leave.

So:

• ja odem = I leave
• ti odeš = you leave
• oni/one/ona odu = they leave

It is a common irregular verb, so the form is worth memorizing.

This is about aspect, which is very important in Croatian.

• otići / odu = perfective: to leave, as a completed action
• odlaziti / odlaze = imperfective: to be leaving, to leave repeatedly, to go away in general

In this sentence, the idea is after the guests have left / once they leave, so the completed action is what matters. That is why odu sounds natural here.

Croatian often drops subject pronouns because the verb ending already tells you who is doing the action.

• peremo = we wash

The ending -mo shows we, so mi is usually unnecessary.

You could say mi peremo tanjure, but that would usually add emphasis, like we wash the plates.

Peremo is the 1st person plural present of prati, meaning to wash.

So:

• perem = I wash
• pereš = you wash
• peremo = we wash

In this sentence, it most naturally means we wash in a general or habitual sense.

Because tanjure is the accusative plural, and tanjuri is the nominative plural.

• tanjuri = plates, as the subject
• tanjure = plates, as the object

Here, the plates are the thing being washed, so Croatian uses the object form:

• Peremo tanjure = We wash the plates

Literally, tanjuri / tanjure means plates.

In English, a translation may sometimes say dishes if the broader idea is washing up after a meal, but the Croatian word here is specifically plates.

If you wanted a more general word like dishes, the wording might be different depending on context.

As written, it most naturally sounds like a general/habitual statement:

• When guests leave, we wash the plates.

That is, this is what we usually do.

If you want to talk about one specific future occasion, Croatian would more often use a future form in the main clause, for example:

• Kad gosti odu, oprat ćemo tanjure.
  = When the guests leave, we’ll wash the plates.

Because Kad gosti odu is a subordinate clause placed before the main clause.

Croatian normally separates that with a comma:

• Kad gosti odu, peremo tanjure.

If you reverse the order, you would usually write:

• Peremo tanjure kad gosti odu.

Yes. Croatian word order is fairly flexible.

You can say:

• Kad gosti odu, peremo tanjure.
• Peremo tanjure kad gosti odu.

Both are correct. The first version puts the time condition first and feels very natural. The second starts with the main action.

Croatian can also move words around for emphasis, but the version you were given is a very standard and clear one.