Poslije tuširanja u kupaonici je velika vlaga.

Because poslije (after) normally takes the genitive case.

The base noun is tuširanje = showering / taking a shower.
Its genitive singular form is tuširanja.

So:

• tuširanje = nominative
• poslije tuširanja = after showering

This is very common in Croatian:

• poslije ručka = after lunch
• poslije posla = after work
• poslije treninga = after training

It is a verbal noun: a noun formed from a verb.

Here it comes from tuširati se = to shower.
So tuširanje means showering or taking a shower.

Croatian often uses verbal nouns where English might use:

• after showering
• after taking a shower
• after the shower

So poslije tuširanja is a very natural Croatian structure.

Because this sentence talks about location, not movement.

With u:

• u + accusative = movement into
• u + locative = being in

Here, the meaning is in the bathroom, so Croatian uses the locative:

• kupaonica = bathroom
• u kupaonici = in the bathroom

Compare:

• Idem u kupaonicu. = I’m going into the bathroom.
• Ja sam u kupaonici. = I’m in the bathroom.

This is mostly about Croatian word order, especially the position of short forms like je.

Je is a clitic, and clitics usually come in the second position of the sentence or clause.

So in:

• Poslije tuširanja u kupaonici je velika vlaga.

the opening phrase is Poslije tuširanja, and then within the rest of the clause, je appears early.

Croatian word order is more flexible than English, so you may also hear:

• Poslije tuširanja velika je vlaga u kupaonici.
• U kupaonici je velika vlaga poslije tuširanja.

These versions all mean basically the same thing, but they place emphasis differently.

Because vlaga is the grammatical subject here, so it is in the nominative case.

• vlaga = humidity, moisture
• velika vlaga = great/high humidity

The adjective must agree with the noun in:

• gender: feminine
• number: singular
• case: nominative

So:

• velika vlaga = correct nominative singular feminine

If it were an object, you might see veliku vlagu, but that is not the role here.

Croatian often does not use a dummy subject like English there.

English says:

• There is a lot of humidity in the bathroom.

Croatian simply says, more literally:

• In the bathroom is high humidity.

So:

• u kupaonici je velika vlaga

works as a natural way to express there is a lot of humidity in the bathroom.

This is very common in Croatian.

Vlaga means moisture, humidity, or dampness.

In this sentence, it refers to the moist, steamy air after a shower.

So depending on context, English translations could be:

• There is a lot of humidity in the bathroom after showering.
• The bathroom is very humid after showering.
• There is a lot of moisture in the bathroom after a shower.

So velika vlaga is not literally only big moisture; it means high humidity / a lot of dampness.

Yes. Nakon also means after, and it also takes the genitive.

So you can say:

• Nakon tuširanja u kupaonici je velika vlaga.

This is also correct.

In many situations, poslije and nakon are interchangeable.
A learner should simply remember that both are followed by the genitive.

No. Kupaonica is the standard Croatian word for bathroom.

You may also hear:

• kupatilo in some neighboring standards/varieties

For standard Croatian, kupaonica is the expected form.

So in Croatian:

• u kupaonici = in the bathroom

Yes, in practice it can overlap a bit in meaning.

Poslije tuširanja literally comes from showering, so the core idea is after showering / after taking a shower.

But in natural translation, English might also say:

• after a shower
• after the shower

depending on context.

So the Croatian phrase is flexible enough that the exact English wording may vary.

Because the sentence does not contain the full verb tuširati se.
It uses the verbal noun tuširanje instead.

Compare:

• Tuširam se. = I am showering.
• Poslije tuširanja... = After showering...

When Croatian turns a verb into a noun like this, the reflexive se usually does not stay with it in the same way.

So poslije tuširanja is normal, not poslije se tuširanja.

Yes, it is understandable and natural.

A Croatian speaker might also choose slightly different wording, for example:

• Poslije tuširanja u kupaonici je puno vlage.
• Nakon tuširanja u kupaonici ima puno vlage.

These may sound a bit more conversational to some speakers, because puno vlage (a lot of humidity/moisture) is very common.

But Poslije tuširanja u kupaonici je velika vlaga is still a correct and natural sentence.