Kad je park tih, slušamo ptice.

Kad means when here.

Yes, kad and kada usually mean the same thing. The difference is mostly style:

• kad = shorter, very common in everyday speech
• kada = a bit fuller, sometimes slightly more formal or careful

So:

• Kad je park tih, slušamo ptice.
• Kada je park tih, slušamo ptice.

Both are correct.

Je is the 3rd person singular present form of the verb biti = to be.

So:

• park je tih = the park is quiet

Croatian often uses forms of biti just like English uses is / am / are.

Here the clause is:

• Kad je park tih = When the park is quiet

Because tih is an adjective describing the noun park.

• park is masculine singular
• the adjective must match it
• so we get tih

That is why:

• tih park = quiet park
• park je tih = the park is quiet

By contrast, tiho is usually an adverb, meaning quietly:

• Govorimo tiho. = We speak quietly.

So in your sentence, tih is correct because it describes park, not the way someone listens.

Croatian does not have articles like English the and a/an.

So:

• park can mean a park or the park
• ptice can mean birds or the birds

The exact meaning depends on context.

That is very normal in Croatian, and English speakers usually need some time to get used to it.

Slušamo means we listen.

The ending -mo tells you the subject is we, so Croatian often leaves out the pronoun mi.

• slušam = I listen
• slušaš = you listen
• sluša = he/she listens
• slušamo = we listen
• slušate = you plural / formal listen
• slušaju = they listen

So:

• Slušamo ptice. = We listen to birds / the birds.

You could also say Mi slušamo ptice, but that adds emphasis to we.

Ptice is the accusative plural here, because it is the direct object of slušamo.

We are listening to what? → ptice

The basic noun is:

• ptica = bird

Its plural is:

• ptice = birds

For this noun, the nominative plural and accusative plural look the same:

• Ptice pjevaju. = The birds are singing.
  Here ptice is the subject.
• Slušamo ptice. = We listen to the birds.
  Here ptice is the object.

So the form is the same, but the function in the sentence is different.

It most naturally sounds like a general or repeated situation:

• When the park is quiet, we listen to the birds.

In other words, whenever the park is quiet, that is what we do.

Croatian present tense can also sometimes refer to what is happening now, depending on context, but without extra context this sentence sounds more like a general fact or routine.

Because Kad je park tih is a subordinate clause, and it comes before the main clause.

Croatian normally separates that kind of clause with a comma:

• Kad je park tih, slušamo ptice.

This is similar to English:

• When the park is quiet, we listen to the birds.

If the main clause comes first, you may also see:

• Slušamo ptice kad je park tih.

In that version, there is usually no comma before kad.

Yes, Croatian word order is fairly flexible.

The neutral order here is:

• Kad je park tih, slušamo ptice.

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

• Slušamo ptice kad je park tih.
• Kad je tih park, slušamo ptice. — possible, but less neutral

Even though word order can move around, not every version sounds equally natural. The original sentence is a very normal, standard way to say it.

No, they are related but not the same.

• slušati = to listen (to)
  This suggests attention or intention.
• čuti = to hear
  This is more about simply perceiving sound.

So:

• slušamo ptice = we listen to the birds
• čujemo ptice = we hear the birds

English also makes this distinction, and Croatian does too.

Yes, park is a masculine noun, and that affects the adjective.

Because park is masculine singular, the adjective must also be masculine singular:

• park je tih

If the noun were feminine or neuter, the adjective form would change:

• ulica je tiha = the street is quiet
• more je tiho = the sea is quiet

So the form tih is directly connected to the gender of park.