Riječ "velik" nije sinonim za riječ "malen", nego joj je antonim.

Croatian does not have articles, so there is no direct equivalent of English the or a/an.

That means riječ can mean word, the word, or sometimes a word, depending on context. In this sentence, English naturally translates it as the word because the speaker is referring to a specific lexical item: velik.

It is used twice for clarity:

Riječ velik = the word velik
za riječ malen = for the word malen

Croatian often repeats the noun when talking explicitly about words as words. That makes it very clear that velik and malen are being discussed as vocabulary items, not used in their normal descriptive sense.

In the original sentence, those words are also marked as quoted items. Here I am using bold instead.

Because they are being cited in their basic dictionary form.

For Croatian adjectives, the usual citation form is masculine singular nominative:

• velik = big
• malen = small

When a word is mentioned as a word, Croatian normally uses that base form. The sentence is not saying that some masculine noun is big or small; it is simply naming the words themselves.

Because the negative form of je is fixed as nije.

So:

• je = is
• nije = is not

This is something learners usually just memorize. Croatian does not normally say ne je.

Because Croatian normally uses nego in the pattern:

nije X, nego Y = not X, but Y

This is the standard way to correct or replace the first idea with the second one.

So:

• nije sinonim ... nego antonim
  = not a synonym ... but an antonym

Ali also means but, but it is more general. In this exact correction pattern after a negation, nego is the natural choice.

Here za means for, as in a synonym for.

A very common Croatian pattern is:

sinonim za + accusative

So:

• sinonim za riječ malen = a synonym for the word malen

The noun riječ is in the accusative singular after za. In this case, the form looks the same as the nominative, so you do not see a visible ending change.

Joj is a short dative pronoun meaning to her or to it.

Here it refers back to riječ malen. Since riječ is a feminine noun in Croatian, the pronoun is feminine too.

So the structure is roughly:

• velik is not a synonym for malen
• rather, it is an antonym to it

Natural English would be:

• but it is its antonym
• or more naturally, but it is the antonym of that word

Because grammatically the sentence is built around the noun riječ.

The speaker is really saying:

• the word velik
• the word malen

So when Croatian later uses a pronoun, it refers back to riječ, and riječ is feminine. That is why the pronoun is joj.

This is very common in Croatian when talking about language itself.

Both joj and je are clitics, which means they are short unstressed words with special placement rules.

In Croatian, clitics usually come near the beginning of the clause, and their internal order is fairly fixed. In that order, a dative pronoun like joj normally comes before the auxiliary je.

So:

• nego joj je antonim is the expected order

This is one of those patterns that becomes natural with exposure, but at first it often feels strange to English speakers.

Because antonim is a noun, and nouns keep their own grammatical gender.

The subject riječ is feminine, but the predicate noun antonim is simply the masculine noun meaning antonym. It does not change to match riječ.

So:

• riječ = feminine noun
• antonim = masculine noun

That is normal in Croatian. Predicate nouns do not automatically agree in gender the way adjectives do.

Yes, but only slightly.

Both malen and mali mean small.
However:

• mali is the more common, neutral everyday word
• malen can sound a bit more expressive, literary, or stylistically marked

In many contexts they overlap, and in this sentence malen is simply being used as an example of a word opposite to velik.