Learn Hardcore Croatian: Torta je za rođendan mog djeda. - The cake is for my grandfather’s birthday.

rođendan is in the accusative because za always takes the accusative when it means “for” (purpose/benefit): za rođendan.
djeda is in the genitive because it expresses possession with rođendan: “whose birthday?” → (of) my grandfather. The possessive pronoun matches: mog is genitive masculine singular of moj.

Why is it djeda, not djedu or djede?

Because this is the genitive singular of djed (“grandfather”): djeda.

djedu is dative/locative (“to/at the grandfather”).
djede is vocative (used to address him directly: “Grandpa!”).
Note: djeda is also the accusative form for animate masculine nouns, but here it’s genitive (possession).

Could I say moga or mojeg instead of mog? Do they differ?

Yes: moga, mojeg, and mog are all standard genitive masculine singular forms of moj.

mog is the short/colloquial form (very common in speech, fine in writing).
moga and mojeg are full forms; style/region may influence which one people prefer.
All three work here: za rođendan moga/mojeg/mog djeda.

Why isn’t it rođendana after za?

Because za governs the accusative, not the genitive. rođendana is genitive singular; after za, use the accusative: za rođendan (correct), not za rođendana.

Can I say za djedov rođendan or za moj djedov rođendan instead?

Yes. Both are natural:

Torta je za djedov rođendan. (“The cake is for grandfather’s birthday.”)
Torta je za moj djedov rođendan. (“The cake is for my grandfather’s birthday.”) This uses the possessive adjective djedov (“grandfather’s”). It’s a bit more compact than rođendan mog djeda; both styles are common.

What’s the difference between za rođendan mog djeda and za mog djeda?

za rođendan mog djeda = for my grandfather’s birthday (the occasion).
za mog djeda = for my grandfather (he is the recipient/beneficiary).
Choose based on whether you mean the event or the person.

Where does je go? Can I move it?

je is a clitic and prefers second position in its clause. Correct placements include:

Torta je za rođendan mog djeda. (neutral)
Za rođendan mog djeda je torta. (fronted prepositional phrase; je follows that phrase)
Avoid placing je at the very end: ✗ Torta za rođendan mog djeda je.

Can I omit je?

Not in a normal full sentence. You might drop it in headlines, labels, or notes (e.g., Torta za rođendan mog djeda as a tag on a box), but for standard sentences, keep je.

How do I express “the” here? There’s no article in torta.

Croatian has no articles. Torta can mean “a cake” or “the cake” depending on context. If you need to be explicit, add a determiner:

Ova torta = this cake
Jedna torta = a/one cake

How do I pronounce rođendan and djeda?

đ (in rođendan) is like a soft English “j” (close to the “j” in “judge”).
dj (in djeda) before e/i sounds like “dy” (many speakers pronounce it almost like đ). So djeda ≈ “dye-dah.”
Rough guide: rođendan ≈ “RO-jen-dahn” (roll the r slightly), stress typically on the first syllable.

Why is it djeda with dj, but rođendan with đ?

In Croatian, đ is its own letter. Some words are spelled with đ (e.g., rođendan), while others use dj (e.g., djed, djeca). Before e/i, dj is often pronounced similarly to đ, but the spelling depends on the specific word. Don’t replace đ with dj in standard writing; people sometimes do it only when they can’t type đ.

What’s the difference between za rođendan and na rođendan?

za rođendan = for/on (someone’s) birthday as an occasion or purpose: Kupujem poklon za rođendan.
na rođendan = to the birthday party/event: Idem na rođendan.
Your sentence uses the purpose/occasion sense.

Can I change the word order?

Yes, for emphasis or flow, while keeping clitic placement in mind:

Torta je za rođendan mog djeda. (neutral)
Za rođendan mog djeda je torta. (emphasis on the occasion)
Torta je za djedov rođendan. (using the possessive adjective)
All are grammatical; the first is the most straightforward.

What are the most useful forms of djed to know?

Nominative (subject): djed — “grandfather”
Genitive (of): djeda — “of (my) grandfather”
Dative/Locative (to/at): djedu
Accusative (see/visit): djeda (same as genitive for animate)
Vocative (addressing): djede
Instrumental (with): djedom

Is torta the same as kolač?

Not exactly. torta is a (usually layered) cake, often for celebrations. kolač is a broader term for pastries/sweets and can mean a piece of cake or smaller baked sweets. For a birthday, torta is the more typical word.

Is djed always used, or can I say deda or dida?

Standard Croatian prefers djed. You’ll hear deda (more common in Serbian and some Croatian regions) and dida/did in Dalmatian/Herzegovinian areas. In formal Croatian, stick with djed; in conversation, regional terms vary.