Learn Hardcore Croatian: U ladici u hodniku držim malu kutiju u kojoj su svi moji ključevi. - In the drawer in the hallway I keep a small box in which all my keys are.

You cannot say u ladici hodniku; that would be ungrammatical because hodniku would be left without a preposition to connect it properly in this meaning.

You can change the order if you like:

U hodniku u ladici držim… – In the hallway, in the drawer, I keep…

but each noun needs its own u.

Why is the word order U ladici u hodniku držim malu kutiju…? Could I also say Držim malu kutiju u ladici u hodniku?

Yes, you can also say:

Držim malu kutiju u ladici u hodniku.

Croatian word order is quite flexible. The choice mainly affects emphasis and flow:

U ladici u hodniku držim malu kutiju…
→ Slight emphasis on where you keep it (location first).
Držim malu kutiju u ladici u hodniku.
→ Slight emphasis on what you keep (the small box) and then you add where.

Both are grammatically correct and mean the same thing in neutral context.

Why is it malu kutiju and not mala kutija?

Because malu kutiju is in the accusative case, used for the direct object of the verb držim (I keep).

Nominative (subject): mala kutija – a small box
Accusative (object): malu kutiju – a small box (as object)

Forms:

mala → malu (feminine, singular, accusative)
kutija → kutiju (feminine, singular, accusative)

Since kutiju is the thing that you are keeping (direct object), it must be in the accusative, and the adjective mala has to agree with it in gender, number, and case, so it becomes malu.

What case is kutiju, and how is it formed from kutija?

Kutiju is the accusative singular of kutija (box).

Declension (singular):

Nominative: kutija (subject)
Genitive: kutije
Dative: kutiji
Accusative: kutiju
Locative: kutiji
Instrumental: kutijom

Here, kutiju is the direct object of držim (I keep), so the accusative is used.

What does u kojoj mean, and why is it kojoj and not kojem or koju?

U kojoj means “in which”.

Kojoj is the feminine locative singular form of the relative pronoun koji (which, who, that). It must:

agree with kutija (feminine, singular)
be in the locative, because it follows u (in)

So:

Feminine nominative: koja – which (one)
Feminine accusative: koju – which (one) (object)
Feminine locative: kojoj – in which, on which, etc., with prepositions

Because the phrase is u kojoj (in which), the locative is required, and it refers back to kutija:

…malu kutiju, u kojoj su svi moji ključevi.
…a small box in which all my keys are.

What case is ključevi, and how is that plural formed from ključ?

Ključevi is nominative plural of ključ (key).

Singular nominative: ključ – key
Plural nominative: ključevi – keys

It is the subject of the verb su (are), so it stands in the nominative.

The plural is formed with -evi:

ključ → ključevi
prijatelj → prijatelji, but
učitelj → učitelji, etc.

Some masculine nouns take -evi in the plural; ključ is one of them.

Why is it moji ključevi and not moja ključevi or moje ključevi?

The possessive adjective moj must agree with ključevi (masculine, plural, nominative).

Basic forms:

Masculine singular nominative: moj – my (book) → moj ključ
Feminine singular nominative: moja – my (box) → moja kutija
Neuter singular nominative: moje – my (child) → moje dijete
Masculine plural nominative: moji – my (keys) → moji ključevi

Since ključevi is masculine plural nominative, you must use moji:

moji ključevi – my keys

What is the role of svi in svi moji ključevi? How is it different from just moji ključevi?

Svi means “all”.

moji ključevi = my keys
svi moji ključevi = all my keys

Grammatically, svi is a pronoun/adjective meaning all and agrees with ključevi (masculine plural nominative):

svi (m. pl. nom)
sve (f. pl. nom; also neuter pl. acc)
etc.

So svi moji ključevi emphasizes that every single one of your keys is in that box.

Why is the verb su used here, and can it move around in the clause?

Su is the 3rd person plural present of biti (to be): they are.

In u kojoj su svi moji ključevi:

ključevi (they) → su (are)

In Croatian, forms like su, sam, si, smo, ste are enclitics (unstressed short forms) and usually occupy the second position in a clause, after the first stressed word.

Here:

u (preposition, unstressed)
kojoj (first stressed word)
then enclitic su

So u kojoj su svi moji ključevi is the natural order. You normally wouldn’t put su at the very end (u kojoj svi moji ključevi su sounds wrong).

What is the difference between držim and imam here? Could I say imam malu kutiju…?

You could say Imam malu kutiju u kojoj su svi moji ključevi, but the nuance changes:

držim = I keep / I store / I hold (somewhere)
→ Focus on where you keep something.
imam = I have / I own
→ Focus on possession, not so much on the storage place.

In the original sentence, držim is better because you are explaining where you keep your keys (in a small box in the drawer in the hallway), not just that you own a small box.

Why is there no ja (I) before držim?

Croatian is a pro-drop language: the subject pronoun is often omitted because the verb ending already shows the person and number.

držim = I keep (1st person singular, clearly marked by -m)
ti držiš = you keep
on/ona drži = he/she keeps

So (Ja) držim malu kutiju… – with ja – is grammatically correct, but in normal speech and writing you simply say:

Držim malu kutiju…

The ja is used only for emphasis or contrast (Ja držim… a ti ne. – I keep it… but you don’t.).