Kad uđemo u vagon, sjednimo blizu prozora da lakše čujemo razglas.

Croatian does not have articles. There is no direct equivalent of English the or a/an.

So:

• u vagon can mean into a carriage or into the carriage
• blizu prozora can mean near a window or near the window
• čujemo razglas can mean hear the announcement system / the PA system

Croatian usually leaves this to context. If you really want to make something more specific, you can use words like:

• taj = that
• ovaj = this
• jedan = one / a certain

Kad and kada both mean when.

In this sentence, kad is simply the shorter, very common form. It is completely normal in everyday Croatian. Kada is a bit fuller and sometimes sounds slightly more formal or careful, but both are correct here.

So these are both possible:

• Kad uđemo u vagon...
• Kada uđemo u vagon...

After time words like kad = when, Croatian often uses the present form, especially with a perfective verb, to refer to a future action.

So kad uđemo u vagon literally looks like when we enter the carriage, but in natural English it often corresponds to when we get into the carriage or once we enter the carriage.

This is not strange if you compare it to English:

• When we get there, let's sit down.

English also often uses a present form after when, not will.

Because Croatian distinguishes between:

• movement into something
• location in something

With u:

• u + accusative = motion into
• u + locative = location in

Here, uđemo means enter / go into, so movement is involved:

• u vagon = into the carriage

Compare:

• Kad uđemo u vagon = When we enter the carriage
• Kad smo u vagonu = When we are in the carriage

Sjednimo is the 1st person plural imperative of sjesti.

That means it expresses:

• let’s sit down

Croatian has an imperative not only for you, but also for we:

• sjedni = sit down (to one person)
• sjednite = sit down (to more than one person / polite)
• sjednimo = let’s sit down

So this sentence is giving a suggestion or proposal that includes the speaker.

This is a very common question.

• sjednimo = let’s sit down
  This is an imperative / suggestion.

• sjednemo = we sit down or, in some contexts, a present form used in subordinate clauses
  It is not the same as let’s...

In this sentence, the speaker is suggesting an action, so sjednimo is the correct choice.

Compare:

• Kad uđemo u vagon, sjednimo blizu prozora. = When we get into the carriage, let’s sit near the window.
• Kad uđemo u vagon, sjednemo blizu prozora. would not express the same clear hortative meaning.

Because blizu normally takes the genitive case.

So:

• prozor = nominative
• prozora = genitive

That is why Croatian says:

• blizu prozora = near the window

You will see the same pattern in many expressions:

• blizu stanice = near the station
• blizu kuće = near the house
• blizu mene = near me

Here da introduces a purpose clause. It means:

• so that
• in order to

So:

• da lakše čujemo razglas = so that we can hear the PA system more easily

This is a very common Croatian pattern:

• Došao sam da pomognem. = I came to help.
• Sjednimo blizu prozora da lakše čujemo. = Let’s sit near the window so that we can hear more easily.

After da in this kind of purpose clause, Croatian normally uses the present tense form, not a future construction.

So Croatian says:

• da čujemo
  not something like
• da ćemo čuti

That is the standard pattern after da when you mean so that we hear / can hear.

In other words:

• da + present is very often how Croatian expresses purpose.

Lakše is the comparative form of the adverb lako.

So:

• lako = easily
• lakše = more easily

In this sentence:

• da lakše čujemo razglas = so that we can hear the PA system more easily

It does not have to mean an explicit comparison like more easily than before. Very often Croatian uses the comparative naturally in situations where English also says more easily.

A close alternative would be:

• da bolje čujemo razglas = so that we can hear the PA system better

But lakše focuses more on ease, while bolje focuses more on quality/clarity.

Yes, the comma is standard here.

The sentence begins with a subordinate clause:

• Kad uđemo u vagon

Then comes the main clause:

• sjednimo blizu prozora...

In Croatian, when a subordinate clause comes before the main clause, it is normally separated by a comma.

So this punctuation is correct:

• Kad uđemo u vagon, sjednimo blizu prozora...

To some extent, yes. Croatian word order is more flexible than English, but not completely free.

The given sentence is very natural:

• Kad uđemo u vagon, sjednimo blizu prozora da lakše čujemo razglas.

You could move some parts for emphasis, but the original order is the most neutral and natural for a learner to use.

For example, blizu prozora could be moved, but that would usually add emphasis or sound more marked:

• Kad uđemo u vagon, sjednimo da lakše čujemo razglas blizu prozora.
  This is much less natural because it can sound as if blizu prozora belongs to the hearing part, not the sitting part.

So yes, Croatian allows flexibility, but in this case the original sentence is the best basic model to follow.