Learn Hardcore Croatian: Moj mlađi brat misli da je miš na računalu isto što i prava životinja. - My younger brother thinks that the mouse on the computer is the same as the real animal.

The change d → đ (mlad → mlađi) is a regular spelling change in Croatian for this adjective; you don’t say mladji in modern standard spelling, but mlađi.

Why is it moj mlađi brat and not moja mlađa brat or moje mlađe brat?

In Croatian, possessives and adjectives must agree with the noun in:

Gender (masculine, feminine, neuter)
Number (singular, plural)
Case (nominative, accusative, etc.)

Here:

brat (brother) is masculine singular nominative.
The possessive moj is the masculine singular nominative form of my.
The adjective mlađi is also masculine singular nominative.

So we get:

moj mlađi brat = my younger brother (masculine singular)

Other options would be:

moja mlađa sestra = my younger sister (feminine singular)
moji mlađi braće / braća – forms used when talking about more than one brother (with some irregularities around braća / braće)

Why is there no word for the or a in Moj mlađi brat misli... or prava životinja?

Croatian has no articles like English the or a/an.

moj mlađi brat can mean my younger brother or my younger brother (the one we know about), depending on context.
prava životinja can be understood as a real animal or the real animal.

Definiteness is usually clear from context, word order, or from using other words (like taj = that, ovaj = this), but there is no direct equivalent of the/a.

What is da doing in misli da je miš na računalu...? Is it like English that?

Yes. Here da is a conjunction meaning that, introducing a subordinate clause:

Moj mlađi brat misli da je miš na računalu...
= My younger brother thinks that the mouse on the computer...

Structure:

Main clause: Moj mlađi brat misli = My younger brother thinks
Subordinate clause: da je miš na računalu isto što i prava životinja
= that the mouse on the computer is the same as a real animal

You normally cannot omit da here the way you sometimes can omit that in English.
You say:

Misli da je u pravu. = He thinks (that) he is right.

but not:

✗ Misli je u pravu.

Why is it misli da je miš... and not misli da miš je...?

The short form of biti (to be) – je – is a clitic. Croatian clitics have a strong tendency to appear in second position in a clause.

In the clause:

da je miš na računalu...

the first word is da, so je naturally comes right after it:

da je miš...

Putting je after miš (da miš je...) sounds wrong in standard Croatian.

Basic pattern:

(Conjunction) + clitic + rest of the clause
- da je miš...
- jer je bio... = because he was...

Why isn’t there any word for it in the part da je miš na računalu isto što i prava životinja?

In English, you say that the mouse on the computer is the same as a real animal, and you might feel there should be an it somewhere.

In Croatian:

The subject of the subordinate clause is miš na računalu (the mouse on the computer).
The verb je (is) already agrees with this subject.
Croatian normally omits subject pronouns (on, ona, ono) unless you need emphasis or clarification.

So you don’t say:

✗ misli da je on miš na računalu isto što i prava životinja (that he/it is the mouse...) – this would change the meaning.

You just say:

misli da je miš na računalu isto što i prava životinja
= he thinks the computer mouse is the same as a real animal.

What case is na računalu, and why is it računalu and not računalo?

The preposition na can be followed by either:

Accusative (movement to/onto something), or
Locative (position on something).

Here, the mouse is on the computer (static location), so na takes the locative:

računalo = the computer (dictionary form, nominative singular, neuter)
na računalu = on the computer (locative singular)

If there were movement onto the computer, you’d use the accusative:

Stavlja miš na računalo. = He is putting the mouse onto the computer. (accusative: računalo)

Is there a difference between računalo, kompjuter, and računar?

They all refer to a computer, but usage is regional and stylistic:

računalo – standard, native Croatian word; common in Croatia, especially in formal or official language.
kompjuter – borrowed from English computer; widely understood and often used in everyday speech.
računar – common and standard in Serbian, Bosnian, and Montenegrin.

In a neutral Croatian sentence like this one, računalo is the most typical choice.

Does miš mean both an animal mouse and a computer mouse in Croatian?

Yes. Miš is used for both:

miš = mouse (the animal)
(računalni) miš or miš za računalo = computer mouse

Context usually makes it clear. In your sentence:

miš na računalu strongly suggests computer mouse, because mice (animals) are not normally on the computer.

To be explicit, you could say:

računalni miš = computer mouse
miš za računalo = mouse for the computer

What does the phrase isto što i mean, and can I replace it with something else?

Isto što i literally means the same (thing) as.

Breakdown:

isto = the same
što = what / that which
i = and, but here functioning like as

So:

isto što i prava životinja = the same as a real animal

Common alternatives:

isto kao prava životinja = the same as a real animal
isti kao prava životinja – when you are agreeing with a masculine noun (e.g., On je isti kao njegov brat. = He is the same as his brother.)

In your sentence, isto što i is very natural and idiomatic.

Why is it prava životinja and not something like pravom životinjom?

Croatian normally uses the nominative for a predicate noun/adjective after biti (to be), just like English:

On je učitelj. = He is a teacher.
To je prava životinja. = That is a real animal.

So:

miš na računalu [je] isto što i prava životinja
= the computer mouse is the same as a real animal

Both miš na računalu and prava životinja are in the nominative, because they are being equated.

The instrumental, e.g. pravom životinjom, is used with other verbs such as postati (to become), baviti se (to deal with / to be engaged in), etc., not with this kind of is the same as construction.

Also note gender agreement:

životinja is a feminine noun.
Therefore the adjective must be feminine: prava (not pravi or pravo).