Ljeti je u gradu velika vrućina, pa radije ostajemo u hladovini.

Ljeti means in summer or during the summer.

It is a common adverbial form used for seasons and times of year. In Croatian, you often use:

• ljeti = in summer
• zimi = in winter
• u proljeće = in spring
• u jesen = in autumn

So ljeti is a natural, fixed way to say in summer.
U ljetu is not the normal choice here.

The structure is basically:

• Ljeti = in summer
• je = is
• u gradu = in the city
• velika vrućina = great heat / intense heat

So literally, it is something like:

In summer, in the city, there is great heat.

Croatian often uses to be in this kind of sentence where English might say:

• It is very hot in the city in summer
• There is intense heat in the city in summer

So je is normal here because the sentence is built around vrućina as a noun.

Because u can take different cases depending on meaning:

• u + accusative = movement into something
  
  • Idem u grad = I’m going into town / to the city
• u + locative = being in a place
  
  • U gradu je vruće = It is hot in the city

Here there is no movement. It describes location, so Croatian uses the locative:

• grad → gradu

Yes, literally it is big heat, but naturally it means:

• great heat
• intense heat
• very hot weather

Croatian often uses velik/velika/veliko in places where English might prefer great, strong, or severe.

So velika vrućina is a very natural Croatian expression.

Because vrućina is a feminine noun.

Adjectives in Croatian must agree with the noun in:

• gender
• number
• case

So:

• vrućina = feminine singular
• therefore velika = feminine singular form of velik

Here pa means something like:

• so
• therefore
• and so
• which is why

It connects the two parts of the sentence:

• Ljeti je u gradu velika vrućina = In summer there is intense heat in the city
• pa radije ostajemo u hladovini = so we prefer to stay in the shade

It shows a consequence or natural result.

Radije means rather or preferably.

In this sentence:

• radije ostajemo = we would rather stay / we prefer to stay

It comes from rado, which means gladly or with pleasure.

A useful comparison:

• rado = gladly
• radije = rather / more willingly
• najradije = most gladly / most of all

Examples:

• Rado idem tamo. = I gladly go there.
• Radije ostajem doma. = I’d rather stay at home.
• Najradije spavam. = Most of all, I like sleeping.

Because ostajemo comes from the imperfective verb ostajati / ostati pair.

Here the sentence describes a usual or repeated action in summer:

• In summer... we prefer to stay in the shade.

For habits, ongoing situations, or repeated actions, Croatian usually prefers the imperfective verb:

• ostajemo = we stay / we are staying / we tend to stay

By contrast, ostanemo is from the perfective verb ostati, which is more about a completed action:

• Ostanemo tamo do večeri. = We end up staying there until evening / We stay there until evening

In this sentence, ostajemo is the natural choice.

Croatian often leaves out subject pronouns because the verb ending already shows the person and number.

Here:

• ostajemo ends in -mo
• that tells you it means we stay

So Croatian does not need mi here.

You could say mi radije ostajemo u hladovini, but that would add emphasis, something like:

• we prefer to stay in the shade

Without mi, the sentence sounds more neutral and natural.

It is locative singular after u, because it means being in the shade, not moving into it.

• base form: hladovina = shade
• locative singular: hladovini

So:

• u hladovini = in the shade

Just like:

• u kući = in the house
• u sobi = in the room

Both can relate to shade/shadow, but they are not always used in exactly the same way.

• hladovina usually means shade, especially a cool shaded place where you stay to avoid the sun
• sjena usually means shadow

So in this sentence, u hladovini is the best choice because it means staying in a cool shady place.

Examples:

• Sjedimo u hladovini drveta. = We are sitting in the shade of the tree.
• Vidim svoju sjenu. = I see my shadow.

Yes. Croatian could also say:

• Ljeti je u gradu jako vruće, pa radije ostajemo u hladovini.

That means:

• In summer it is very hot in the city, so we prefer to stay in the shade.

The difference is mainly stylistic:

• jako vruće = very hot
• velika vrućina = intense heat / great heat

The version with vrućina sounds a bit more noun-based and descriptive.

Yes, Croatian word order is fairly flexible.

This sentence is natural as written:

• Ljeti je u gradu velika vrućina, pa radije ostajemo u hladovini.

But other orders are possible, depending on emphasis, for example:

• U gradu je ljeti velika vrućina, pa radije ostajemo u hladovini.
• Radije ostajemo u hladovini, jer je ljeti u gradu velika vrućina.

The original order sounds natural and neutral.
Putting ljeti first nicely sets the time frame right away.

Yes, in standard Croatian it is normal to put a comma before pa when it connects two clauses like this.

Here the sentence has two parts:

• Ljeti je u gradu velika vrućina
• pa radije ostajemo u hladovini

So the comma helps show the connection between them.