Learn Hardcore Croatian: Najprije završimo zadatak, a tek onda gledamo seriju. - First we finish the task, and only then we watch a series.

najprije can sound slightly more neutral or careful, sometimes a bit more “ordered”: first this, then that.
prvo is very common in speech and sometimes feels a bit more casual.

For a learner, you can safely treat them as synonyms here.

What exactly is završimo here – present tense or an imperative like “let’s finish”?

Formally, završimo is the 1st person plural present of the perfective verb završiti.

However, in Croatian, the 1st person plural present form is also used as a “let’s …” imperative:

Završimo zadatak. = Let’s finish the task.

So in this sentence:

Najprije završimo zadatak…
is best understood as
First, let’s finish the task…

It’s a suggestion/instruction including the speaker: “you and I / we should do this now.”

Why is it završimo (perfective) but gledamo (imperfective)? Is there a reason for that aspect choice?

Yes, this is a classic aspect contrast in Slavic languages.

završiti (here: završimo) is perfective – it focuses on completing the task, the result:
- završimo zadatak = let’s complete/finish the task (bring it to an end).
gledati (here: gledamo) is imperfective – it focuses more on the activity / process of watching:
- gledamo seriju = we watch / we’ll watch the series (as an activity).

So the idea is:

First, we complete one whole thing (perfective) – the task.
Then we engage in another activity (imperfective) – watching the series.

You could also say pogledamo seriju (perfective: “watch it through to the end”), but gledamo seriju is a normal choice when talking about watching episodes/TV as an activity.

Is gledamo present or future here? Why not ćemo gledati?

Grammatically, gledamo is present tense.

However, Croatian very often uses the present tense for planned or scheduled future actions, especially when the sequence is clear from context:

Najprije završimo zadatak, a tek onda gledamo seriju.
= First, let’s finish the task, and only then we watch / we’ll watch the series.

You can say:

Najprije ćemo završiti zadatak, a tek onda ćemo gledati seriju.

This is also correct and sounds a bit more explicit and a bit more formal.
The original sentence is more natural and conversational.

What does a mean here? Why not just use i?

Both a and i are coordinating conjunctions, but they’re not identical:

i = and, simply adding information.
a = and / but, often with a slight contrast, opposition, or step in the sequence.

In this sentence:

Najprije završimo zadatak, a tek onda gledamo seriju.

a signals a transition or contrast:

First do this, and only then (as something different, later) do that.

If you use i:

Najprije završimo zadatak, i tek onda gledamo seriju.

it’s still understandable, but a is more idiomatic here because it highlights the contrast in order: task first, series later.

What does tek add in a tek onda? What’s the difference between onda and tek onda?

onda = then
tek (in this context) = only, not until

So:

onda gledamo seriju = then we watch the series.
tek onda gledamo seriju = only then / not until then do we watch the series.

tek adds the idea of restriction or delay:
you must first complete the previous action, and only after that can the second one happen.

So the full clause:

a tek onda gledamo seriju
≈ and only then do we watch the series.

Is a tek onda one fixed phrase, or can I change the order (e.g. a onda tek gledamo seriju)?

a tek onda is a very common chunk, but it’s not completely rigid.

Possible variants:

a tek onda gledamo seriju – very natural, standard.
a onda tek gledamo seriju – also possible; tek is now a bit more strongly tied to gledamo (“then we only then watch”).

Subtle nuance:

a tek onda gledamo seriju
focuses more on the time point: only at that point in time.
a onda tek gledamo seriju
can feel a bit more like “then and not earlier do we watch”, with emphasis on the action.

For a learner, a tek onda is the safest and most idiomatic choice here.

Why is it zadatak but seriju? What cases are these?

Both zadatak and seriju are in the accusative singular (direct object case), but they decline differently because of their gender and noun type.

zadatak (task)
- Masculine noun.
- Nominative sg: zadatak
- Accusative sg (for an inanimate masculine noun): zadatak (same as nominative).
- So: završimo zadatak = we finish the task.
serija (series, TV show)
- Feminine noun ending in -a.
- Nominative sg: serija
- Accusative sg: seriju (the -a changes to -u).
- So: gledamo seriju = we watch the series.

This -a → -u change in the accusative is a regular pattern for many feminine nouns in -a (e.g. škola → školu, knjiga → knjigu).

Is this sentence more spoken or written? Is it informal?

Najprije završimo zadatak, a tek onda gledamo seriju. is:

Completely natural in spoken Croatian.
Neutral in tone – not slang, but also not very formal.
It could also appear in informal writing (e.g. messages, emails, dialogues in books).

A more formal or more carefully explicit version would be:

Najprije ćemo završiti zadatak, a tek onda ćemo gledati seriju.

But even that is acceptable in speech; it just sounds a bit more “complete” and less colloquial.

Can I say Najprije ćemo završiti zadatak, a tek onda ćemo gledati seriju instead? Does it change the meaning?

Yes, you can say:

Najprije ćemo završiti zadatak, a tek onda ćemo gledati seriju.

This is grammatically correct and means essentially the same thing:

First we will finish the task, and only then will we watch the series.

Differences:

The original sentence uses present/imperative-like forms and sounds more direct and conversational.
The ćemo + infinitive construction is the standard future tense and sounds a bit more explicit and a touch more formal or careful.

In normal conversation, the original sentence is slightly more natural and shorter.

Is the comma before a necessary?

Yes, in standard Croatian writing, you normally put a comma before coordinating conjunctions like a, i, ali when they join two independent clauses (each with its own subject and verb):

Najprije završimo zadatak, a tek onda gledamo seriju.
(clause 1: Najprije završimo zadatak; clause 2: tek onda gledamo seriju)

So the comma here is correct and expected in standard writing. In very casual texting people sometimes skip commas, but for proper writing, keep it.