Learn Hardcore Croatian: Subotom ujutro uključim usisavač i usisavam stan dok ostali spavaju. - On Saturday mornings I turn on the vacuum cleaner and vacuum the apartment while the others sleep.

Subotom is the instrumental plural form of subota and is commonly used as a time expression meaning “on Saturdays / on Saturday(s) in general.”

subota – nominative singular: Saturday (as a subject: Subota je moj omiljeni dan.)
subotu – accusative singular: usually on Saturday in the sense of one specific Saturday (Vidimo se u subotu.)
subotom – instrumental plural: on Saturdays (habitually, regularly)

In this sentence, you’re describing a habit, so Croatian prefers Subotom.

Could I also say Svake subote ujutro uključim usisavač instead of Subotom ujutro?

Yes.

Subotom ujutro = On Saturday mornings (as a habit) – a bit shorter and very natural.
Svake subote ujutro = Every Saturday morning – explicitly stresses every Saturday.

Both are correct and very natural for habitual actions. The difference is just a slight emphasis: svake subote makes the frequency more explicit.

Why is ujutro written as one word? Can you also say u jutro?

In everyday language, ujutro is usually written as one word and functions as an adverb meaning in the morning / in the mornings.

ujutro – adverb: Subotom ujutro… = On Saturday mornings…
jutro – noun: Jutro je hladno. = The morning is cold.
u jutro with a space is grammatically possible but is rare and sounds more like “into the morning (hours)” in some contexts. For the normal “in the morning” time expression, use ujutro (one word).

Why is it uključim and not uključujem here?

The verbs are from two different aspectual partners:

uključiti – perfective (focus on the whole, completed act of switching on)
uključivati – imperfective (focus on ongoing / repeated activity)

In the sentence Subotom ujutro uključim usisavač, the speaker is talking about a repeated, but each time completed event in a routine:
each Saturday morning there is one clear moment when they switch on the vacuum cleaner.

Croatian often uses the perfective present in such repeated but complete sequences, especially in descriptions of what normally happens:

Kad dođem kući, uključim TV i skuham kavu.

You could say Subotom ujutro uključujem usisavač, but that shifts the focus more to the activity in progress or general ability, and sounds a bit less natural for this kind of neat routine description. Uključim feels more like a crisp, step-by-step sequence.

Why is the second verb usisavam, not usisam?

Here you have the imperfective verb usisavati:

usisavati – imperfective: to be vacuuming, to vacuum (as an ongoing or repeated action)
usisati – perfective: to vacuum up / completely, to finish vacuuming

In i usisavam stan, the focus is on the action in progress / activity that takes some time: you are vacuuming the apartment while others sleep. The imperfective fits this idea of an ongoing action overlapping with another action (others sleeping).

If you said i usišem stan, it would emphasize finishing the whole apartment, more like “…and I (manage to) get the whole apartment vacuumed.” That would be possible, but it slightly changes the nuance.

Is there a connection between usisavač and usisavam?

Yes, they are related:

usisavati – to vacuum (literally: to suck in)
usisavač – vacuum cleaner (literally: “the thing that vacuums / sucks in”)

So usisavač is the device, and usisavam is I vacuum, the 1st person singular present of usisavati. The pattern noun -ač often marks a person or thing that performs the action of the verb.

Why is it usisavam stan, not something like usisavam stanom?

The verb usisavati is transitive and takes a direct object in the accusative:
Koga? Što? (Whom? What?)

usisavam stan – stan is accusative singular: I vacuum the apartment

Stanom is instrumental singular (with / by means of the apartment), which doesn’t fit here. You’re performing the action on the apartment, so it must be in the accusative.

What does ostali mean exactly, and why is that form used?

Ostali means “the others / the rest (of them)”.

Grammatically, ostali here is:

masculine plural, nominative, used as a noun: the others

Croatian often uses adjective forms as nouns:

mladi – the young (people)
bogati – the rich (people)
ostali – the others / everyone else

If the group were all female, you could say ostale (feminine plural). Here, ostali is the default masculine plural, used whenever the group is mixed or unspecified.

Could I say dok drugi spavaju instead of dok ostali spavaju?

Yes, dok drugi spavaju is also grammatically correct and understandable.

ostali = the others / the rest (of the household, of us, etc.) – clearly refers to everyone else except the speaker (and possibly except some understood subgroup).
drugi = other / different – can mean other people in general, but is a bit more vague: “while other people are sleeping.”

In this context, ostali sounds slightly more like “the rest of the people in my home / family”, which fits nicely with usisavam stan.

Why is spavaju used, not a past or future tense after dok?

Dok means “while / as long as” and usually takes the present tense to express two actions happening at the same time:

usisavam stan – I vacuum the apartment
dok ostali spavaju – while the others are sleeping

Even though English often uses a continuous form (are sleeping), Croatian simply uses the present tense for ongoing actions in such clauses. The simultaneity is expressed by dok + present tense.

Can I change the word order, for example: Ujutro subotom uključim usisavač…?

Yes. Croatian word order is fairly flexible, especially for adverbials of time. All of these are possible:

Subotom ujutro uključim usisavač…
Ujutro subotom uključim usisavač…
Uključim usisavač subotom ujutro…

They all mean the same. The most neutral and common in this kind of sentence is probably the original Subotom ujutro…, but the others are not wrong; they just change the rhythm and emphasis slightly (for example, putting Ujutro first can emphasize the time of day a bit more).