Kad čistim kupaonicu, prvo obrišem prašinu, a onda spužvom perem pločice.

Kad means when here, often with the sense of whenever.

So the sentence can be understood as:

• When I clean the bathroom...
• or Whenever I clean the bathroom...

Yes, you can also say kada.
Kad is the shorter, very common everyday form, while kada is a bit fuller and sometimes slightly more formal or careful in style.

So both are correct:

• Kad čistim kupaonicu...
• Kada čistim kupaonicu...

Because Croatian often uses the present tense for habitual or general actions, just like English does in sentences such as When I clean the bathroom, I first...

Here čistim does not mean only I am cleaning right now. It means something like:

• when/whenever I clean
• when I do bathroom cleaning

So this is a general routine, not necessarily something happening at this exact moment.

Because kupaonicu is in the accusative case.

The basic dictionary form is kupaonica = bathroom.
But after the verb čistiti when the bathroom is the direct object, Croatian uses the accusative:

• čistim kupaonicu = I clean the bathroom

For many feminine nouns ending in -a, the accusative singular ends in -u:

• kupaonica → kupaonicu
• kuhinja → kuhinju
• soba → sobu

Prvo means first.

It is an adverb that helps organize the sequence of actions:

• prvo obrišem prašinu = first I wipe the dust
• a onda... = and then...

Its position is fairly flexible, but the sentence as given is very natural.
You may also hear:

• Kad čistim kupaonicu, obrišem prvo prašinu...

That is also possible, but prvo obrišem prašinu is a very normal, clear way to say it.

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

• brisati → brišem = imperfective
  meaning the action as ongoing, repeated, or uncompleted
• obrisati → obrišem = perfective
  meaning to wipe something off / finish wiping / remove it completely

So:

• brišem prašinu suggests I am wiping dust or I wipe dust as an activity
• obrišem prašinu suggests I wipe the dust off as a completed step

In this sentence, obrišem fits well because the speaker sees this as one finished stage in the cleaning routine.

This is a very common learner question.

In Croatian, the present form of a perfective verb often does not describe something happening right now in a simple standalone way. But it can be used very naturally in:

• future meaning
• habitual sequences
• clauses with words like kad meaning when/whenever

So in a sentence like:

• Kad čistim kupaonicu, prvo obrišem prašinu...

obrišem means something like:

• I wipe the dust off
• I get the dust wiped off as a regular completed step whenever that situation happens

This is normal Croatian usage.

Because prašinu is also in the accusative case, as the direct object of obrišem.

The dictionary form is prašina = dust.
In the sentence:

• obrišem prašinu = I wipe off the dust

Again, this is the same feminine -a → -u pattern in the accusative singular:

• prašina → prašinu

Spužvom is the instrumental singular of spužva = sponge.

Croatian often uses the instrumental case without a preposition to express the tool or means used to do something.

So:

• spužvom perem pločice = I wash the tiles with a sponge

This is like saying:

• olovkom pišem = I write with a pen
• nožem režem = I cut with a knife

The ending -om is a common instrumental singular ending for feminine nouns:

• spužva → spužvom

This is another aspect question.

• prati → perem = imperfective
• oprati → operem = perfective

Perem pločice presents the action as an activity or process:
I wash the tiles

Operem pločice would stress the action as completed:
I wash the tiles completely / I get the tiles washed

In this sentence, perem sounds natural because the speaker is describing the cleaning routine and focusing on the action itself. If you said operem pločice, that would also be possible, but it would put more emphasis on finishing that step.

So the difference is mainly one of viewpoint:

• perem = the washing activity
• operem = the completed result

Here pločice is the accusative plural, because it is the direct object of perem.

The dictionary form is pločica = tile.
Plural:

• pločice = tiles

For this noun, the nominative plural and accusative plural look the same, so the form does not change visibly here.

So:

• perem pločice = I wash the tiles

Both can often be translated as and then, but they are not always identical in feeling.

• a onda often marks the next step in a sequence, sometimes with a slight sense of transition or contrast
• i onda is more simply and then

In this sentence, a onda sounds very natural because the speaker is moving from one step to the next:

• first dusting
• then washing the tiles

So a onda is a good choice for a routine with stages.

Not completely. Croatian word order is more flexible than English word order, although some versions sound more natural than others.

The original sentence is very natural:

• Kad čistim kupaonicu, prvo obrišem prašinu, a onda spužvom perem pločice.

You could also hear variations such as:

• Kad čistim kupaonicu, prvo obrišem prašinu, a onda perem pločice spužvom.
• Prvo obrišem prašinu, a onda spužvom perem pločice kad čistim kupaonicu.

But moving things around changes the focus and sometimes makes the sentence sound less smooth or less neutral.

For example:

• spužvom perem pločice puts a little more attention on the tool
• perem pločice spužvom sounds slightly more neutral in some contexts

So the word order is flexible, but the original version is a very good standard model to learn from.

There are two commas for two different reasons.

1. After kupaonicu

   • Kad čistim kupaonicu is a subordinate time clause
   • when that clause comes before the main clause, it is separated by a comma

2. Before a onda

   • a is a coordinating conjunction joining two parts of the sentence
   • in this kind of structure, a comma is normally used before it

So the punctuation helps show the structure:

• Kad čistim kupaonicu = time clause
• prvo obrišem prašinu = first main action
• a onda spužvom perem pločice = next main action