Learn Hardcore Croatian: Cijeli vikend učimo hrvatski bez stresa. - The whole weekend we are learning Croatian without stress.

cijeli vikend literally just puts cijeli (whole) + vikend (weekend) together, and the context tells you if it’s a whole weekend, the whole weekend, or just all weekend.
Native speakers never add an extra word to mark the; the language doesn’t work that way.

So cijeli vikend can be understood as the whole weekend depending on context, even though there’s no separate word for the.

What form is cijeli here? Why not cijela or cijelo?

cijeli is an adjective meaning whole / entire, and it must agree with the noun vikend in:

gender (masculine)
number (singular)
case (accusative here)

vikend is a masculine noun, so the matching masculine accusative singular form of cijeli is cijeli.

Other forms you might see with different nouns:

cijela kuća – the whole house (feminine)
cijelo jutro – the whole morning (neuter)

Here: cijeli vikend = masculine singular accusative.

What case is vikend in, and why is there no preposition like for (“for the whole weekend”)?

vikend is in the accusative singular.

Croatian often uses the accusative without a preposition to express duration of time:

cijeli dan – (for) the whole day
tri sata – (for) three hours
cijeli vikend – (for) the whole weekend

In English you often need for, but in Croatian you normally just put the time expression in the accusative.

What exactly does učimo mean here? Is it “we study” or “we are studying”? Can it also refer to the future?

učimo is:

present tense
1st person plural (we)
of the verb učiti (to learn, to study)

In English, it can correspond to:

we study Croatian (habitual)
we are studying Croatian (right now)

The sentence Cijeli vikend učimo hrvatski bez stresa is most naturally understood as a planned or ongoing activity over the weekend. Because of cijeli vikend, Croatian present tense can also imply a near-future arrangement:

This weekend we are learning Croatian (all weekend).

So yes, present tense in Croatian often covers both English simple present and present continuous, and with a time expression it can refer to a planned future.

Why is there no mi (“we”)? Is it optional?

The mi is optional because Croatian is a pro‑drop language: personal pronouns are usually left out when the verb ending already shows the person.

učimo already tells you it’s we (1st person plural).
mi učimo hrvatski is correct but normally used only for emphasis, like “we (as opposed to someone else) are learning Croatian”.

So the neutral, natural version is without mi: Učimo hrvatski.

Why is it hrvatski and not hrvatski jezik?

Both are possible:

učimo hrvatski
učimo hrvatski jezik

In everyday speech, people very often drop the word jezik (language) when it’s obvious from context that you are talking about the language:

pričam engleski – I speak English (language)
uče njemački – they learn German

Adding jezik is more explicit or formal, but in this sentence učimo hrvatski is completely natural and standard.

What form/case is hrvatski here, and why does it end in -i?

Here hrvatski is:

an adjective used as a noun (meaning Croatian [language])
masculine singular accusative

It’s the direct object of učimo:

učimo (što?) hrvatski – we are learning (what?) Croatian

For a typical adjective:

nominative masculine singular: hrvatski
accusative masculine singular (for inanimate nouns): also hrvatski

So hrvatski (accusative) looks the same as hrvatski (nominative) in this case.

Note: Hrvatska (with a capital H and -a) is the country (Croatia), feminine form.

What does bez stresa literally mean, and what case is stresa?

bez stresa literally means without stress.

bez (without) is a preposition that always takes the genitive case.
The base noun is stres (stress).
Genitive singular of stres is stresa.

So:

nominative: stres
genitive: stresa
phrase: bez stresa – without stress / stress‑free

Can I change the word order, like Učimo hrvatski cijeli vikend bez stresa? Does that change the meaning?

You can change the word order; Croatian word order is fairly flexible. Some common variants:

Cijeli vikend učimo hrvatski bez stresa.
Učimo hrvatski cijeli vikend bez stresa.
Cijeli vikend učimo hrvatski, bez stresa.

All have basically the same core meaning. Differences are mostly in rhythm and emphasis:

Starting with cijeli vikend slightly highlights the duration: the whole weekend is the frame for what follows.
Starting with Učimo hrvatski first highlights the activity (learning Croatian), then adds when and how.

For a learner, consider them equivalent; the original is a very natural, neutral sentence.

What is the difference between učiti hrvatski and naučiti hrvatski?

They form an aspect pair:

učiti – imperfective: to be learning / to study (the process)
naučiti – perfective: to learn / to have learned (reach a result)

Examples:

Cijeli vikend učimo hrvatski. – We are learning / studying Croatian the whole weekend (focus on the activity).
Jednog dana naučit ćemo hrvatski. – One day we will learn Croatian (and know it) (focus on completing the learning).

In your sentence, the focus is clearly on the ongoing activity during the weekend, so učiti is the natural choice.

Is vikend a borrowed word, and does it decline like other Croatian nouns?

Yes, vikend is borrowed from English weekend, but it behaves like a normal Croatian masculine noun and declines:

nominative: vikend
genitive: vikenda
dative/locative: vikendu
accusative: vikend
instrumental: vikendom

In cijeli vikend, it’s in the accusative singular (duration of time).
Another example:

Radim cijeli vikend. – I’m working all weekend.
Bez vikenda sam umoran. – Without a weekend I’m tired. (Here vikenda is genitive after bez.)

How would I say “We will be learning Croatian all weekend” using an explicit future tense?

You can keep the same structure and just switch to a future form of učiti:

Cijeli vikend ćemo učiti hrvatski bez stresa.
or, with clitic after the verb:
Cijeli vikend učit ćemo hrvatski bez stresa.

Both mean We will be learning Croatian all weekend without stress.

In everyday speech, though, people often just use the present as in the original sentence, especially when the time reference (cijeli vikend) is clear.