Kad djeca zaspu, usisat ću blagovaonicu i hodnik.

Yes. Kad and kada both mean when.

• Kad is the shorter form and is very common in everyday Croatian.
• Kada is a bit fuller and can sound slightly more formal or careful.

In this sentence, Kad djeca zaspu means when the children fall asleep.

Because zaspati and spavati do not mean the same thing.

• zaspati = to fall asleep
• spavati = to sleep / to be sleeping

So:

• Kad djeca zaspu = When the children fall asleep
• Kad djeca spavaju would mean something more like When the children are sleeping

The original sentence focuses on the moment after they fall asleep, not just the state of being asleep.

Zaspu is the 3rd person plural present form of zaspati.

So:

• ja zaspim = I fall asleep
• ti zaspiš = you fall asleep
• oni/one/ona zaspu = they fall asleep

Even though it is a present-tense form, after kad it refers to the future here: when the children fall asleep.

Because Croatian normally does not use the future tense with će after kad in this kind of time clause.

Instead, Croatian uses a present form, often from a perfective verb, to express a future event:

• Kad djeca zaspu, ... = When the children fall asleep, ...

This is similar to English, which also says When the children fall asleep, not When the children will fall asleep.

Because Croatian often leaves out subject pronouns when they are already clear from the verb form.

Here, ću shows 1st person singular, so I is already understood:

• usisat ću = I will vacuum

You could add ja if you want emphasis:

• Ja ću usisati blagovaonicu i hodnik.

That would sound more like I will vacuum the dining room and hallway with extra emphasis on I.

The infinitive is usisati.

In the future tense, Croatian uses the auxiliary ću. When ću comes after an infinitive ending in -ti, the final -i is dropped:

• usisati + ću → usisat ću

But if the auxiliary comes first, the full infinitive stays:

• Ja ću usisati blagovaonicu i hodnik.

So both patterns are normal:

• Usisat ću ...
• Ću usisati ... is not used by itself because ću is a clitic and normally needs something before it, such as ja or another word.

Yes. That is a perfectly natural sentence.

Croatian word order is fairly flexible, and both versions work:

• Kad djeca zaspu, usisat ću blagovaonicu i hodnik.
• Ja ću usisati blagovaonicu i hodnik kad djeca zaspu.

The difference is mostly about focus:

• starting with Kad djeca zaspu emphasizes the time condition first
• starting with Ja ću usisati... emphasizes the action more directly

Both words are direct objects of usisati, so they are in the accusative case.

But nouns change differently depending on gender and type:

• blagovaonica is feminine

  • nominative: blagovaonica
  • accusative: blagovaonicu

• hodnik is masculine inanimate

  • nominative: hodnik
  • accusative: hodnik

So hodnik looks unchanged, while blagovaonica changes from -a to -u.

Djeca means children. It goes with a plural verb here:

• Djeca zaspu = The children fall asleep

This is one of those words you need to learn as a pair:

• dijete = child
• djeca = children

So even though the form may look unusual to an English speaker, it is the normal word for children.

Yes, but the meaning would be a bit different because of aspect.

• usisati = perfective
  A completed action: vacuum up / vacuum completely / finish vacuuming
• usisavati = imperfective
  An ongoing, repeated, or habitual action: be vacuuming / vacuum regularly

In this sentence, the speaker means a single completed future action, so usisati is the natural choice:

• Kad djeca zaspu, usisat ću... = When the children fall asleep, I’ll vacuum...

Croatian does not have articles like English a/an/the.

So blagovaonicu i hodnik can mean:

• the dining room and the hallway
• or sometimes just a dining room and a hallway

The exact meaning depends on context. In a sentence like this, English usually uses the, but Croatian simply does not need an article.

Because Kad djeca zaspu is a subordinate time clause placed before the main clause.

Croatian normally separates that fronted subordinate clause with a comma:

• Kad djeca zaspu, usisat ću blagovaonicu i hodnik.

This is similar to English punctuation in sentences like:

• When the children fall asleep, I’ll vacuum the dining room and the hallway.