Njena temperatura je danas niža nego jučer.

Both njena and njezina mean her in Croatian.

• njena is very common in everyday speech.
• njezina is also correct and is often preferred in more formal standard Croatian.

Here, njena agrees with temperatura, which is:

• feminine
• singular
• nominative

So the full phrase njena temperatura means her temperature.

Temperatura is the subject of the sentence, so it is in the nominative case.

The sentence structure is basically:

• Njena temperatura = the subject
• je = is
• danas niža nego jučer = the rest of the predicate

Because temperatura is the thing being described, it stays in its basic dictionary form here.

Je is the 3rd person singular present form of biti (to be).

So here it means is.

• Njena temperatura je niža = Her temperature is lower

Croatian often uses forms of biti just like English uses to be when linking the subject to an adjective.

Niža is the feminine singular form of the comparative adjective niži (lower).

It has to agree with temperatura, which is feminine singular.

So:

• masculine: niži
• feminine: niža
• neuter: niže

Because temperatura is feminine, you need niža.

The base adjective is nizak, which means low.

Its comparative is niži = lower.

So the pattern is:

• nizak = low
• niži / niža / niže = lower

In this sentence, you see the feminine form niža because it describes temperatura.

Nego means than in comparisons.

So:

• niža nego jučer = lower than yesterday

Croatian commonly uses nego after comparatives:

• veći nego... = bigger than...
• brži nego... = faster than...
• niža nego... = lower than...

With comparisons, Croatian often uses nego when what follows is really a shortened clause or adverbial idea.

Here, nego jučer means something like:

• lower than it was yesterday

Since jučer (yesterday) is an adverb of time, nego is the natural choice.

You could use od with a noun phrase, for example:

• niža od jučerašnje temperature = lower than yesterday’s temperature

But with just jučer, nego is the normal option.

Because jučer is an adverb, not a noun here.

Adverbs do not change for case, gender, or number.

So:

• danas = today
• jučer = yesterday

Both stay the same in the sentence.

Danas means today and functions as a time adverb.

It tells you when her temperature is lower.

So the sentence contrasts two times:

• danas = today
• jučer = yesterday

That gives the meaning:

• Her temperature is lower today than yesterday.

No, Croatian word order is fairly flexible.

The original sentence:

• Njena temperatura je danas niža nego jučer.

is natural and neutral.

You could also say:

• Danas je njena temperatura niža nego jučer.
• Njena temperatura danas je niža nego jučer.

These all mean basically the same thing, but the emphasis shifts slightly.

• Starting with danas highlights today
• Starting with njena temperatura highlights her temperature

The version you were given sounds very natural.

Yes. A very natural alternative is:

• Danas ima nižu temperaturu nego jučer.

Literally: Today she has a lower temperature than yesterday.

Another common possibility is:

• Temperatura joj je danas niža nego jučer.

Literally: Her temperature is lower today than yesterday, but with joj meaning to her / her in this possessive-like structure.

Croatian often allows several natural ways to express the same idea.

You certainly can:

• Njezina temperatura je danas niža nego jučer.

That is fully correct.

The difference is mostly stylistic:

• njena = very common in everyday use
• njezina = also correct, often felt as a bit more formal or more strictly standard

So learners should understand both.

A helpful approximation is:

• nj sounds like the ny in canyon
• e is a clear short eh
• a is ah

So njena is roughly NYEH-na.

The first sound is important: nj is treated as a single consonant sound in Croatian.

Roughly:

• ju = yoo
• č = like ch in chop
• er = a short rolled or tapped r with e

So approximately: YOO-cher.

A closer Croatian pronunciation has:

• a sharper č
• a trilled or tapped r

Also note that jučer means yesterday, while sutra means tomorrow.

In this context, yes, that is the natural interpretation.

Temperatura can mean:

• temperature in general
• body temperature
• air/weather temperature
• the temperature of an object

But when you say njena temperatura about a person, Croatian speakers will normally understand body temperature.

Yes. In English, we naturally say lower temperature, and Croatian works the same way.

So:

• niža temperatura = lower temperature
• viša temperatura = higher temperature

This is completely normal Croatian usage.

Word by word, it is:

• Njena = her
• temperatura = temperature
• je = is
• danas = today
• niža = lower
• nego = than
• jučer = yesterday

So the literal structure is:

• Her temperature is today lower than yesterday

Natural English rearranges this as:

• Her temperature is lower today than yesterday.