Čim se vratimo, raspakirat ćemo torbe i oprati sve što je bilo unutra.

After time conjunctions like čim, Croatian normally uses the present tense to refer to future events. English does something similar: we say as soon as we return, not as soon as we will return.

So Čim se vratimo literally uses a present form, but it means as soon as we return in the future.

Because the verb meaning to return / come back is vratiti se.

• vratiti se = to return, come back
• vratiti = to return something, give something back

So without se, the meaning would change.

Se is a clitic, and Croatian clitics usually go in second position in their clause. Since this clause starts with čim, the clitic comes right after it:

Čim se vratimo

That word order is normal and expected.

Croatian often leaves out subject pronouns because the verb ending already shows who the subject is.

Here, vratimo and ćemo both show we.
A separate mi could be added for emphasis, but it is not necessary.

This is the standard spelling of the future with many infinitives ending in -ti. When the clitic form of htjeti comes directly after the infinitive, the infinitive usually drops its final i in writing:

• raspakirati + ćemo → raspakirat ćemo

So raspakirati ćemo is not the standard written form here.

Because the auxiliary ćemo is understood for both verbs.

The structure is basically:

raspakirat ćemo torbe i oprati sve...

In other words, one ćemo covers both raspakirati and oprati. Since ćemo is not repeated after oprati, the second verb stays in the full infinitive form.

Because oprati is perfective and means to wash completely / finish washing.
Prati is imperfective and focuses more on the process or repeated action.

In this sentence, the speaker is talking about a one-time future action with a clear result, so the perfective oprati is the natural choice.

The same idea applies to vratiti se and raspakirati: they also point to completed actions.

Torbe is the direct object of raspakirati, so it is in the accusative plural.

For a feminine inanimate noun like torba, the accusative plural looks the same as the nominative plural:

• nominative plural: torbe
• accusative plural: torbe

So the form does not change here, even though the case does.

After general words like sve, nešto, or ono, Croatian normally uses što as the relative pronoun.

So:

sve što je bilo unutra = everything that was inside

Using a form of koji here would sound unnatural in standard Croatian.

Because bilo agrees with sve, and sve is treated grammatically as neuter singular.

So the agreement is:

• sve → neuter singular
• therefore bilo

Even though sve refers to many things in meaning, grammatically it takes singular neuter agreement.

Unutra is an adverb meaning inside or in there.

In što je bilo unutra, it tells you where something was. It is not an adjective and does not change form for gender, number, or case.