Ljeti najradije sjedim u hladovini.

Ljeti is an adverb meaning in summer or during the summer.

The basic noun is ljeto = summer, but in this sentence Croatian uses the adverbial form ljeti to express time.

Compare:

• ljeto = summer
• ljeti = in summer

This is similar to other seasonal expressions:

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

So Ljeti tells you when the action usually happens.

Najradije is the superlative form of rado.

The pattern is:

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

In Croatian, rado / radije / najradije are often used with verbs to show preference.

So:

• Rado sjedim = I gladly sit / I like sitting
• Radije sjedim = I would rather sit
• Najradije sjedim = I most like to sit / I prefer sitting most

In natural English, this sentence is often translated with prefer or like best, even though Croatian uses this adverb.

Both are possible, but they are slightly different in style.

• Najradije sjedim u hladovini sounds very natural and compact.
• Najviše volim sjediti u hladovini also works and means something like I like sitting in the shade the most.

Croatian often prefers rado / radije / najradije + verb to express liking or preference about actions.

So this sentence is a very idiomatic way to say that sitting in the shade is your preferred thing to do in summer.

Sjedim is the 1st person singular present tense of sjediti = to sit.

So:

• sjedim = I sit / I am sitting

In this sentence, it has a habitual meaning:

• I sit
• I usually sit
• I like to sit

Croatian present tense is often used for general habits and regular behavior, not only for what is happening right now.

Because Croatian usually does not need the subject pronoun when the verb ending already shows the person.

Here, sjedim clearly means I sit, so ja is unnecessary.

You could say:

• Ja ljeti najradije sjedim u hladovini.

But that would usually add emphasis, something like:

• I, in summer, prefer sitting in the shade.

The version without ja is the most neutral and natural one.

Because Croatian uses different cases after u depending on the meaning:

• u + accusative = movement into something
• u + locative = being in a place

Here there is no movement into the shade; the speaker is already sitting there. So Croatian uses the locative:

• u hladovini = in the shade

Compare:

• Sjedim u hladovini. = I am sitting in the shade.
• Idem u hladovinu. = I am going into the shade.

Hladovini is the locative singular of hladovina.

Basic form:

• hladovina = shade, shady area

In this sentence:

• u hladovini = in the shade

So the noun changes because it follows u in a location meaning.

Yes, absolutely.

Both are natural:

• u hladu
• u hladovini

The difference is slight:

• hlad often means shade or coolness in a broad sense
• hladovina often suggests a shady place/area, especially under trees or somewhere sheltered from the sun

In everyday speech, both can work here. U hladovini sounds a little more like in a shady spot.

No, Croatian word order is fairly flexible, but different orders change the emphasis.

The given sentence:

• Ljeti najradije sjedim u hladovini.

This is natural and neutral, with ljeti setting the time first.

Other possible orders:

• Najradije ljeti sjedim u hladovini.
  Emphasizes najradije a bit more.
• U hladovini ljeti najradije sjedim.
  Stronger emphasis on u hladovini.

So the original order is a very normal way to say it, but it is not the only possible order.

It can mean both, depending on context.

Croatian present tense often covers both:

• I sit
• I am sitting

In this sentence, because of ljeti and najradije, the meaning is clearly habitual/general, not something happening only right now.

So here sjedim is best understood as:

• I usually sit
• I prefer to sit
• I like sitting

No. This is an important verb pair in Croatian.

• sjediti = to be sitting, to sit
  
  • ongoing state
• sjesti = to sit down
  
  • completed action of taking a seat

So:

• Sjedim u hladovini. = I am sitting in the shade.
• Sjednem / sjest ću u hladovinu. = I sit down / I will sit down in the shade.

In your sentence, sjedim is correct because it describes the state or activity, not the action of sitting down.

It can match either one in English.

Ljeti is a broad time expression, so depending on context you might translate it as:

• in summer
• during the summer
• in the summertime

All of these fit the Croatian sentence well.