Nemoj pomiješati lozinku i korisničko ime.

It is a negative command addressed to one person.

Croatian often leaves out subject pronouns, so the implied subject here is ti:
(Ti) nemoj pomiješati lozinku i korisničko ime.

So grammatically, this is basically Don’t mix up the password and the username.

Because this is the normal Croatian way to make a negative imperative for one person:

• nemoj + infinitive = don’t do X
• nemojte + infinitive = don’t do X to several people or in polite/formal speech

So:

• Nemoj pomiješati... = Don’t mix up...
• Nemojte pomiješati... = Don’t mix up... (plural/formal)

A form like Ne pomiješaj is not the usual standard way to say this.

This is about aspect, which is very important in Croatian.

• miješati = imperfective
• pomiješati = perfective

Here, pomiješati is natural because the speaker is warning against one complete mistake: mixing up the password and username.

Very roughly:

• Nemoj pomiješati... = Don’t end up mixing them up
• Ne miješaj... = Don’t keep mixing them up / don’t be mixing them up

In a one-time warning, the perfective pomiješati fits very well.

Lozinku is in the accusative singular because it is a direct object of the verb.

The base form is:

• lozinka = password (nominative)

In the accusative singular, a feminine noun like this changes to:

• lozinku

So:

• lozinka → nominative
• lozinku → accusative

That is why you see lozinku here.

It actually is in the accusative too, but for many neuter singular nouns, the accusative looks exactly like the nominative.

The noun here is:

• ime = name (neuter)

So:

• nominative: korisničko ime
• accusative: korisničko ime

Same form, different function.

The adjective korisničko agrees with ime, and in this case it also looks the same in nominative and accusative neuter singular.

Yes, basically.

It is a two-word phrase:

• korisničko = user-related / belonging to a user
• ime = name

So korisničko ime literally means user name, which is the standard Croatian term for username.

Croatian usually writes this as two words, not one.

Yes.

It links the two objects:

• lozinku i korisničko ime = the password and the username

With a verb like pomiješati, this means to confuse one with the other / mix them up.

So the sense is not just listing two things, but warning not to mistake them for each other.

Yes. Croatian word order is fairly flexible.

For example, you could also say:

• Nemoj pomiješati korisničko ime i lozinku.

That still means the same thing.

The original order may just reflect what the speaker wants to mention first. In Croatian, word order can change for:

• emphasis
• rhythm
• focus
• style

But in a simple sentence like this, both orders are normal.

Because Croatian usually does not need subject pronouns when the verb already makes the meaning clear.

Here, nemoj already shows that the command is directed at one person. So ti is understood automatically.

You could say:

• Ti nemoj pomiješati lozinku i korisničko ime.

But that usually sounds more emphatic, as if you are specifically singling someone out.

You would use nemojte instead of nemoj:

• Nemojte pomiješati lozinku i korisničko ime.

So:

• nemoj = singular, informal
• nemojte = plural or polite/formal singular

This is exactly like many other Croatian commands.

Yes. A common alternative is zamijeniti.

For example:

• Nemoj zamijeniti lozinku i korisničko ime.

That means something like Don’t confuse/swap the password and the username.

The difference is roughly:

• pomiješati = mix up, confuse
• zamijeniti = exchange, swap, mistake one for the other

In computer or login contexts, zamijeniti can sound especially natural, but pomiješati is also correct.

A helpful rough English-style guide is:

NE-moy po-mi-YE-sha-ti LO-zin-ku ee ko-RISH-nich-ko EE-me

A few useful pronunciation points:

• j is pronounced like English y
• š is pronounced like sh
• č is pronounced roughly like ch
• i is pronounced like ee

So:

• nemoj ≈ NE-moy
• pomiješati ≈ po-mi-YE-sha-ti
• korisničko ≈ ko-RISH-nich-ko
• ime ≈ EE-me

The exact pronunciation depends a little on accent and region, but that guide is close enough for a learner.