Razumijem nju bolje nego njega.

Because nju and njega are object forms, while ona and on are subject forms.

• on / ona = he / she as the subject
• njega / nju = him / her as the direct object

The verb razumjeti (to understand) takes a direct object, so Croatian uses the accusative case here:

• Razumijem nju = I understand her
• Razumijem njega = I understand him

So ona and on would be wrong in this sentence.

They are in the accusative case, because they are the direct objects of razumijem.

One small detail is that njega looks like a genitive form too. That is normal: for masculine animate words in Croatian, the accusative often has the same form as the genitive.

So here:

• nju = accusative singular of ona
• njega = accusative singular of on

Even though njega has the same shape as a genitive form, in this sentence its function is accusative.

Razumijem means I understand.

It is the 1st person singular present tense of razumjeti.

The ending -em tells you the subject is I, so Croatian does not need the pronoun ja unless you want emphasis.

• Razumijem nju = I understand her
• Ja razumijem nju = I understand her (with extra emphasis on I)

In standard Croatian, razumijem is the normal form.

Because Croatian usually leaves subject pronouns out when the verb already shows the person clearly.

Here, razumijem already means I understand, so ja is unnecessary.

Croatian often works like this:

• Govorim. = I am speaking.
• Znam. = I know.
• Razumijem. = I understand.

You can add ja if you want contrast or emphasis, but normally you do not need it.

Bolje means better, and here it is an adverb.

That matters because it describes how the understanding happens:

• Razumijem nju bolje = I understand her better

It does not describe a noun, so Croatian does not use an adjective here.

This is the comparison:

• dobro = well
• bolje = better

So the structure is basically:

• understand + better + than ...

Nego means than in comparisons.

Here it connects the two things being compared:

• bolje nego njega = better than him

So the sentence compares two objects of understanding:

• I understand her
• better than
• I understand him

In Croatian, nego is very common after comparatives like bolje, especially when what follows is a pronoun or an implied clause.

English I understand her better than him is ambiguous. It can mean either:

1. I understand her better than I understand him
2. I understand her better than he understands her

Croatian helps distinguish these meanings by case.

Razumijem nju bolje nego njega means:

• I understand her better than I understand him

That is because njega is an object form.

If you wanted the other meaning, you would use a subject form such as on:

• Razumijem je bolje nego on.
• Razumijem je bolje nego što je on razumije.

Those mean:

• I understand her better than he does

So the case choice makes the meaning clearer.

Croatian has both clitic (unstressed) pronouns and full/stressed pronouns.

For these pronouns:

• nju = full/stressed form of her
• ju / je = clitic form of her
• njega = full/stressed form of him
• ga = clitic form of him

The full forms are natural here because the sentence is making a contrast between her and him.

Also, after nego, Croatian normally uses a full form, not a clitic. So njega is expected here.

That is why this version sounds balanced and clear:

• Razumijem nju bolje nego njega.

A version with a clitic in the first half is possible in some contexts, for example Razumijem je bolje nego njega, but the full form nju makes the contrast stronger.

Yes. Croatian word order is fairly flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• Razumijem nju bolje nego njega. = fairly neutral
• Nju razumijem bolje nego njega. = stronger emphasis on her
• Njega razumijem lošije nego nju. = same idea from the opposite angle

Because Croatian marks grammatical roles with case, word order does not have to do all the work that English word order does.

Still, some orders sound more natural than others depending on context.

Yes, usually.

• je / ju is a weaker, unstressed clitic form
• nju is a stronger, stressed full form

So:

• Razumijem je. = I understand her. (more neutral)
• Razumijem nju. = I understand her. (more contrastive or emphatic)

In your sentence, the speaker is comparing her and him, so the stronger forms fit very well.

Nju is pronounced roughly like nyoo.

The sound nj in Croatian is a single sound, similar to:

• Spanish ñ
• the ny in canyon for many English speakers

So:

• nju ≈ nyoo
• njega ≈ NYE-ga or NYE-gah, depending on how narrowly you approximate it in English

This nj sound is very common in Croatian, so it is worth getting used to early.

Yes. It is a natural and clear way to say that the speaker understands her better than him.

What makes it sound especially natural is:

• correct object case: nju, njega
• comparative adverb: bolje
• natural comparison word: nego
• full pronouns for contrast

So for a learner, this is a very good model sentence for practicing:

• object pronouns
• comparison
• emphasis through full pronoun forms