Learn Hardcore Croatian: Njegova ideja je različita od moje.

Because it agrees with the subject ideja, which is feminine singular. Predicate adjectives in Croatian agree in gender and number with the subject:

masculine: različit
feminine: različita
neuter: različito
plural: različiti/različite (masc/fem)

What case is moje here, and why?

It’s feminine genitive singular. The preposition od (“from/than” in comparisons) always takes the genitive, so “my” must be in genitive to match the understood noun (ideje): od moje (ideje).

Why can moje stand alone without repeating ideja?

Croatian often omits a repeated noun when it’s clear from context. Od moje is a shorthand for od moje ideje. The form of moje tells you which gender/number/case the omitted noun would have.

Why not say od moja or od moj?

Because od requires genitive. Moja is nominative feminine singular; moj is nominative masculine singular. You need the genitive feminine singular: moje.

Could I say od mene (“from me”) instead of od moje?

No. That would compare his idea to a person (“His idea is different from me”), which is odd. You want to compare like with like: his idea vs. my idea → od moje (ideje). You could also say the fuller od moje ideje.

Is od the right word for “different from/than”? What about nego?

With nouns/pronouns, use od: različit(a) od X. Use nego mainly when comparing whole clauses or after negation:

Različito je nego što sam mislio. (It’s different than I thought.)
Nije isto, nego drugačije. (It’s not the same, but different.)

Can I use synonyms like drugačija or drukčija?

Yes. Both are common and natural:

Njegova ideja je drugačija/drukčija od moje. All take od + genitive in this structure. You can also use the verb:
Njegova se ideja razlikuje od moje.

Why not use the adverb različito?

Različito is an adverb (“differently”). Here you need an adjective that describes the noun ideja, so use različita (feminine singular adjective).

Can the word order change?

Yes, for emphasis or style:

Njegova ideja je različita od moje. (neutral)
Njegova ideja je od moje različita. (focus on the contrast)
Različita je njegova ideja od moje. (focus on “different”) The meaning stays the same.

How would it look with a masculine or neuter noun?

Masculine: Njegov plan je različit od mog(a)/mojeg(a) (plana).
Neuter: Njegovo mišljenje je različito od mog(a)/mojeg(a) (mišljenja). For masculine/neuter genitive singular, use mog(a)/mojeg(a); for feminine, use moje.

How would I say “Her idea is different from mine”?

Use the feminine possessive:

Njezina (standard) or njena (very common) → Njezina/Njena ideja je različita od moje.

Is je always required? What about jest?

In standard Croatian, the copula je is required. Jest is a more emphatic or formal variant:

Njegova ideja jest različita od moje. (adds emphasis to “is”) You may see je move for clitic placement (e.g., Različita je…), but don’t omit it in standard prose.

What exactly does od do to the case of the following word?

Od governs the genitive. With the full noun, you’d say:

od moje ideje (genitive of ideja is ideje). When the noun is omitted, the possessive alone still appears in genitive: od moje.

How do I pronounce tricky letters like nj and j here?

nj in njegova is one sound, like Spanish ñ in “señor.”
j is always like English “y”: je = “ye,” moje = “mo-ye.”
od has a clearly pronounced final d.

Is there a comparative like “more different,” e.g., različitija?

Forms like različitiji/različitija exist but are rare and usually avoided (as in English). To express degree, use modifiers:

znatno/vrlo/sasvim različita (significantly/very/completely different)
Or switch to drugačija/drukčija, which pairs naturally with degree words.

What’s the fully explicit version without ellipsis?

Njegova ideja je različita od moje ideje. The shorter od moje is more natural when the noun is obvious from context.