Kad završim s mrvicama, usisat ću i tepih u dnevnom boravku.

Both kad and kada mean when.

• kad is shorter and very common in everyday speech and writing.
• kada is a bit fuller and can sound slightly more formal or emphatic.

So:

• Kad završim...
• Kada završim...

are both correct here.

This is about aspect.

• završiti = to finish (perfective: one completed action)
• završavati = to be finishing / to finish repeatedly or over time (imperfective)

In this sentence, the speaker means when I finish completely, so Croatian uses the perfective verb:

• Kad završim... = When I finish...

Using završavam would not fit as naturally here, because the idea is a single completed action that happens before the next one.

This is very normal in Croatian.

After words like kad (when), Croatian often uses the present tense form of a perfective verb to talk about a future completed action.

So:

• Kad završim s mrvicama... literally looks like When I finish with the crumbs...
• but it really means When I have finished / once I finish with the crumbs...

English also does something similar:

• When I finish, I’ll... not
• When I will finish, I’ll...

So Croatian Kad završim... is perfectly natural for future meaning.

Mrvicama is in the instrumental plural.

That is because the expression is:

• završiti s + instrumental

Here s means something like with in the sense of to be done with / to finish dealing with.

So:

• mrvice = crumbs
• s mrvicama = with the crumbs / dealing with the crumbs

The whole part means something like:

• Kad završim s mrvicama... = When I’m done with the crumbs...

Završiti s nečim means to finish with something, to be done with something, or to finish dealing with something.

In this sentence, it does not necessarily mean the crumbs themselves are doing the action. It means the speaker is finished cleaning them up or handling that task.

So the sense is:

• When I’m done with the crumbs
• After I finish cleaning up the crumbs

It is a very natural Croatian way to express this idea.

Both are correct.

Croatian future tense can be formed in two common ways:

• ću usisati
• usisat ću

When the auxiliary ću comes after the infinitive, the final -i of the infinitive drops:

• usisati → usisat ću

So these are equivalent:

• Usisat ću tepih.
• Ću usisati tepih.
  This one is not correct, because ću cannot stand in first position like that by itself.
• Ja ću usisati tepih.
• Usisat ću tepih.

In your sentence, usisat ću is completely normal.

The dictionary form is usisati.

In the sentence, you see usisat ću, which is just the future construction where the final -i drops before the enclitic ću.

So:

• dictionary form: usisati = to vacuum, to suck up
• future: usisat ću = I will vacuum

This happens with many verbs in this future pattern.

Here i means also / too.

So:

• usisat ću i tepih = I’ll vacuum the carpet too / as well

The position of i matters. It usually comes before the word it focuses on.

Here it focuses on tepih, meaning that the carpet is an additional thing the speaker will vacuum.

So the idea is something like:

• first the crumbs,
• then also the carpet.

Tepih is in the accusative singular, which for this noun looks the same as the nominative.

It is the direct object of usisati:

• usisati što? = to vacuum what?
• tepih = the carpet

Because tepih is an inanimate masculine noun, its accusative singular is the same as its nominative singular.

So:

• nominative: tepih
• accusative: tepih

Because this expresses a location, not motion.

Croatian uses:

• u + locative for being in a place
• u + accusative for going into a place

Here the carpet is located in the living room, so:

• u dnevnom boravku = in the living room

Case details:

• dnevni boravak = living room
• locative singular: dnevnom boravku

So this phrase is exactly what you would expect for a static location.

Both can refer to a living room, depending on context.

• dnevni boravak is very common for living room/lounge
• dnevna soba is also common and often understood the same way

In many situations they are interchangeable.
Dnevni boravak can sometimes feel a bit more like the room where people spend time during the day, while dnevna soba is more literally day room / living room, but in everyday use the difference is often small.

The base noun is:

• mrvica = a crumb

Plural:

• mrvice = crumbs

In the sentence, you have the instrumental plural:

• s mrvicama = with the crumbs

So the forms are:

• singular nominative: mrvica
• plural nominative: mrvice
• plural instrumental: mrvicama

Because the sentence describes completed, one-time actions in sequence.

1. završiti = finish completely
2. usisati = vacuum up / do the vacuuming as a complete action

The meaning is not about an ongoing process, but about one action ending and another one happening after it.

So Croatian naturally chooses perfective verbs here:

• Kad završim... = once I finish
• usisat ću... = I’ll vacuum

This gives the sentence a clear sense of progression from one completed task to the next.

Yes. Croatian word order is flexible, although some versions sound more natural in certain contexts.

For example, you could also say:

• Kad završim s mrvicama, usisat ću i tepih u dnevnom boravku.
• Usisat ću i tepih u dnevnom boravku kad završim s mrvicama.

Both are grammatical. The original version is natural because it first sets the time condition:

• When I’m done with the crumbs...

and then gives the main action:

• ...I’ll vacuum the carpet too.

So the original order is a very normal, clear choice.