U srpnju je u gradu često sparina, a navečer je zrak još uvijek sparan.

Because after u meaning in, Croatian normally uses the locative case for location/time.

• srpanj = July
• u srpnju = in July

So here srpnju is the locative singular form of srpanj.

Compare:

• u srpnju = in July
• u gradu = in the city

Both are locative after u.

Gradu is also in the locative singular.

The noun is:

• grad = city

After u when it means in/inside/at a place, Croatian uses the locative:

• u gradu = in the city

So in this sentence, both u srpnju and u gradu are examples of u + locative.

Sparina is a noun. It means something like:

• mugginess
• oppressive heat
• stuffy, humid weather

So često sparina is literally something like there is often mugginess.

Croatian often uses a noun where English might prefer an adjective or a full expression like:

• it is often muggy
• there is often oppressive humidity

That is why the first clause uses sparina.

Because sparan is an adjective, while sparina is a noun.

• sparina = mugginess, oppressive humidity
• sparan = muggy, stuffy, oppressive

In the second clause, the subject is zrak (air), so the adjective must agree with it:

• zrak = masculine singular
• sparan = masculine singular adjective

So:

• zrak je sparan = the air is muggy/stuffy

Because zrak needs an adjective here, not a noun.

• zrak je sparan = the air is muggy
• sparina je = there is mugginess / it is muggy

You can think of it like this:

• noun pattern: sparina je
• adjective pattern: zrak je sparan

Croatian switches naturally between these two ways of expressing similar ideas.

Navečer means in the evening or toward evening.

It is an adverb, so it functions as a time expression.

Examples:

• Navečer čitam. = I read in the evening.
• Navečer je hladnije. = It is cooler in the evening.

In your sentence:

• a navečer je zrak još uvijek sparan = and in the evening the air is still muggy

Because each clause has its own form of the verb to be:

1. U srpnju je u gradu često sparina
2. a navečer je zrak još uvijek sparan

In both clauses, je is the 3rd person singular present of biti (to be).

Croatian often uses je in weather-style statements like this:

• Vruće je. = It is hot.
• Sparina je. = It is muggy / there is mugginess.
• Zrak je sparan. = The air is muggy.

Because je is a clitic in Croatian. Clitics usually go in the second position in their clause.

So in:

• U srpnju je u gradu često sparina

the first unit is U srpnju, and je comes right after it.

Likewise:

• a navečer je zrak još uvijek sparan

after the initial element navečer, the clitic je appears early.

This is very normal Croatian word order.

Yes. Croatian word order is fairly flexible, as long as it still sounds natural and the clitic rules are respected.

For example, variants like these are possible:

• U gradu je u srpnju često sparina.
• U srpnju je često sparina u gradu. (less neutral, more marked depending on context)

The original sentence is natural because it presents the time first, then the place, then the general condition.

Često means often. It is an adverb of frequency.

It modifies the idea of the whole situation:

• često sparina = it is often muggy / there is often mugginess

Croatian adverbs can move around somewhat, but the placement here is very natural:

• U srpnju je u gradu često sparina.

Još uvijek means still.

• još by itself can mean still, yet, or more/additional
• još uvijek is a clearer, fuller way to say still

So:

• zrak je još uvijek sparan = the air is still muggy

Yes, još alone is often possible:

• zrak je još sparan

But još uvijek sounds a bit more explicit and natural in many contexts.

Here a is a linking word with a mild contrast, something between and and while/but.

So:

• ..., a navečer ... means something like
  • ..., and in the evening ...
  • ..., while in the evening ...
  • ..., but in the evening ... (very mild contrast)

It is not as strongly adversative as ali.

Because Croatian has no articles.

So:

• u gradu can mean in the city
• zrak can mean air or the air, depending on context

You understand definiteness from the situation, context, and normal usage, not from separate words like the or a.

Yes, absolutely.

A very natural alternative would be:

• U srpnju je u gradu često sparno, a navečer je zrak još uvijek sparan.

Here:

• sparno je = it is muggy

This uses the adverb/predicative form sparno instead of the noun sparina.

So Croatian has several natural ways to express this idea:

• sparina je = there is mugginess / it is muggy
• sparno je = it is muggy
• zrak je sparan = the air is muggy

Yes, sparina is feminine.

A good clue is the ending:

• nouns ending in -a are very often feminine in Croatian

Its basic form is:

• sparina = nominative singular

If you saw it in other cases, the endings would change like other feminine nouns:

• sparine
• sparini
• sparinu, etc.

In this sentence, it is in the nominative singular because it functions as the predicate noun after je.

It sounds like a general statement about typical conditions.

Clues:

• U srpnju = in July
• često = often
• navečer = in the evening
• još uvijek = still

So the sentence describes a usual pattern:

• in July, it is often muggy in the city
• and in the evening, the air is still muggy

This is not the most natural way to describe one single event.