Learn Hardcore Croatian: Za prijavu na tu mrežu trebaš e-mail i lozinku. - For registration on that network you need an email and a password.

za always takes the accusative case when it means for (the purpose of).
The noun prijava (registration, application, login) in accusative singular becomes prijavu.

So the pattern is:

za + accusative noun → for (the purpose of) doing X
- za prijavu = for registration / for signing up
- za učenje = for studying
- za putovanje = for travelling

You could also say da se prijaviš na tu mrežu (so that you sign up for that network), which uses the verb prijaviti se, but the given sentence uses a noun phrase instead of a verb phrase.

Why exactly is it prijavu and not prijava?

Because prijava is in the accusative case after za.

Nominative (dictionary form): prijava
Accusative singular (feminine -a noun): prijavu

The preposition za (for) requires the accusative when it expresses goal/purpose:

za prijavu (for registration)
za vožnju (for driving)
za večeru (for dinner)

So za prijava would be ungrammatical; the noun must change form to accusative: prijavu.

What does prijava actually mean here—signing up or logging in?

In tech contexts, prijava can mean both:

signing up / creating an account (registracija)
logging in / logging on (login)

The exact meaning depends on context or on how that website/app normally uses the term. In many interfaces you’ll see:

Prijava – Log in
Registracija – Sign up

In this standalone sentence, it’s best taken as “signing up / registering”, but it could also mean “for logging in to that network; you need an email and password.”

Why is it na tu mrežu and not u tu mrežu?

Both na and u can translate as on / in / into, but they’re used differently:

na is often used for platforms, services, networks, websites:
- na Facebooku (on Facebook)
- na toj mreži (on that network)
- na internetu (on the Internet)
u is more “physically in / inside a space”:
- u sobi (in the room)
- u kutiji (in the box)
- u gradu (in the city)

For online networks, na mreži / na tu mrežu is the natural choice.
So na tu mrežu roughly feels like onto that network / on that network in English.

What case is tu mrežu, and why does it look like that?

tu mrežu is in the accusative singular:

Nominative: ta / tu mreža (that network)
Accusative: *tu mrežu (that network as an object / target)

Both words change (or show) accusative:

tu – feminine accusative singular demonstrative (this/that)
mreža → mrežu – feminine noun ending in -a, accusative -u

We use accusative because:

na
- accusative = movement / direction (onto / to something)
  - na tu mrežu (onto that network)
And the whole phrase is governed by za prijavu na tu mrežu (for registration on to that network).

What case is lozinku, and why isn’t it lozinka?

Lozinku is the accusative singular of lozinka (password).

Nominative: lozinka (password)
Accusative: lozinku (a password as a direct object)

In trebaš e-mail i lozinku:

The verb trebati (“to need”) takes a direct object in the accusative.
So both needed things are in accusative:
- e-mail (indeclinable here)
- lozinku (from lozinka)

You could also see lozinka declined in other cases:

genitive: lozinke
dative/locative: lozinki
instrumental: lozinkom

Is e-mail also in the accusative? Why doesn’t its form change?

Yes, e-mail here functions as an accusative direct object, just like lozinku.

However:

Many speakers treat e-mail as indeclinable (it keeps the same form in all cases), especially in informal speech.
In more careful or traditional usage, it can be declined as a masculine noun:
- Nominative: e-mail
- Accusative: e-mail
- Genitive: e-maila, etc.

In this sentence, you simply see e-mail unchanged in form, but grammatically it’s the object you need, so its role is accusative, even if the word shape doesn’t show it.

What exactly does trebaš mean, and how is it conjugated?

Trebaš is the 2nd person singular of the verb trebati.

infinitive: trebati – to need, to be necessary
ja trebam – I need
ti trebaš – you (sg.) need
on/ona/ono treba – he/she/it needs
mi trebamo – we need
vi trebate – you (pl./formal) need
oni/one trebaju – they need

So trebaš e-mail i lozinku = you need an email and a password.

Note: trebati can also be used impersonally (like to be necessary):

Treba mi e-mail. – I need an email. (An email is needed to me.)

Could I say “To sign up to that network you must have an email and password” with morati instead of trebati?

Yes, but there is a nuance difference:

trebati = to need
- Trebaš e-mail i lozinku. – You need an email and a password.
morati = must / have to
- Moraš imati e-mail i lozinku. – You must have an email and a password.

Both are natural. Trebaš is a bit more neutral/practical (“for this action, these things are required”), while moraš sounds more like a rule or obligation.

Is the word order fixed, or can I say “Trebaš e-mail i lozinku za prijavu na tu mrežu”?

You can absolutely change the word order:

Za prijavu na tu mrežu trebaš e-mail i lozinku.
Trebaš e-mail i lozinku za prijavu na tu mrežu.

Both are correct and common. Croatian word order is relatively flexible; basic rules:

The part you want to emphasize can be moved toward the beginning.
Starting with za prijavu na tu mrežu puts focus on the purpose.
Starting with trebaš e-mail i lozinku puts focus on what you need.

What’s the difference between trebaš and trebate?

Both are forms of trebati:

trebaš – 2nd person singular, informal “you” (ti)
- Used with friends, family, people you’re on first-name terms with.
trebate – 2nd person plural, either:
- plural you (talking to more than one person), or
- polite/formal singular you (Vi) when addressing one person politely.

So for a polite version of this sentence, you’d say:

Za prijavu na tu mrežu trebate e-mail i lozinku.
(You [formal] need an email and a password to sign up to that network.)

Can I leave out tu and just say “Za prijavu na mrežu trebaš e-mail i lozinku”?

Yes.

na tu mrežu – to that network (a specific, known network)
na mrežu – to a/the network in a more general sense

Croatian doesn’t have articles (a, the), so tu acts like that and makes it specific. Without tu, the sentence still works, but it doesn’t point to a particular one as clearly.

You might use tu mrežu if:

You’ve already mentioned the network.
You’re pointing at it on a screen or referring to a specific service everyone in the conversation knows.