Ona szepcze mi do ucha, że pociąg zaraz odjeżdża.

That’s true: Polish often leaves out subject pronouns when the verb already makes the subject clear.

• szepcze already means he/she/it whispers
• so Szepcze mi do ucha... would also be a perfectly natural sentence

Including ona usually adds one of these effects:

• emphasis: She is the one whispering
• contrast: she, not someone else
• clarity: if the context needs the subject spelled out

So ona is not required here, but it is grammatical and natural.

szepcze is the:

• 3rd person singular
• present tense
• of the verb szeptać = to whisper

So:

• ja szepczę = I whisper
• ty szepczesz = you whisper
• on/ona/ono szepcze = he/she/it whispers

In this sentence, it matches ona.

mi means to me and is in the dative case.

The verb pattern here is:

• szeptać komuś = to whisper to someone

So:

• szepcze mi = she whispers to me
• szepcze mu = she whispers to him
• szepcze jej = she whispers to her

mi is the common short form of mnie in the dative.

• mi is usually used in normal speech
• mnie can sound more emphatic in some contexts

So szepcze mi is the most natural choice here.

This is a fixed, very common expression:

• szeptać komuś do ucha = to whisper in someone’s ear / into someone’s ear

Literally, do means to, into, toward, and it takes the genitive case.

So:

• ucho = ear
• do ucha = into the ear / to the ear

Even though English often says in my ear, Polish normally says do ucha with this verb.

Because the preposition do requires the genitive case.

The noun is:

• ucho = ear

Its genitive singular is:

• ucha

So:

• ucho = nominative
• do ucha = genitive after do

This is one of the most important things to watch in Polish: prepositions often force a specific case.

Because że introduces a subordinate clause, like English that.

The sentence has two parts:

• Ona szepcze mi do ucha = She whispers in my ear
• że pociąg zaraz odjeżdża = that the train is leaving soon

In Polish, a comma is normally required before że.

So the comma here is standard punctuation, not optional.

że means that and introduces reported speech or reported information.

So:

• szepcze mi do ucha, że... = whispers in my ear that...

It connects the main clause to what is being whispered.

Very common patterns include:

• mówi, że... = says that...
• myślę, że... = I think that...
• wiem, że... = I know that...

Because pociąg is the subject of the clause pociąg zaraz odjeżdża.

The subject normally appears in the nominative case, which is the dictionary form.

So:

• pociąg = train
• pociąg odjeżdża = the train is leaving

Nothing is forcing a different case here, so nominative is used.

zaraz often means:

• soon
• in a moment
• right away
• any second now

In this sentence, it suggests immediacy:

• pociąg zaraz odjeżdża = the train is leaving very soon / the train is about to leave

So it is stronger and more urgent than a simple soon in many contexts.

Polish often uses the present tense for something that is happening very soon, especially when it is:

• already imminent
• scheduled
• practically in progress

So:

• pociąg zaraz odjeżdża literally looks like the train is leaving soon
• in natural English, depending on context, this may be the train is about to leave

The verb odjeżdża is from odjeżdżać, an imperfective verb. Here it presents the departure as an event that is just about to happen or is unfolding in real time.

Yes, but the nuance changes.

• odjeżdża = is leaving / is about to leave / leaves
• odjedzie = will leave

With zaraz, both can be possible in some contexts, but:

• zaraz odjeżdża sounds very immediate, as if the departure is imminent
• zaraz odjedzie sounds a bit more like a completed future event

In this sentence, odjeżdża fits the urgent whisper very well.

Yes. Polish word order is fairly flexible, though not completely free.

This sentence could appear in different orders, for example:

• Szepcze mi do ucha, że pociąg zaraz odjeżdża.
• Mi szepcze do ucha, że pociąg zaraz odjeżdża.
• Do ucha mi szepcze, że pociąg zaraz odjeżdża.
  (more marked or stylistic)

The original order is natural, but changing the order changes what is emphasized.

For example:

• Ona at the start emphasizes she
• mi earlier can emphasize to me
• zaraz before odjeżdża highlights urgency

Literally, it is just to the ear. But because the sentence already has mi = to me, Polish naturally understands whose ear is meant.

So:

• szepcze mi do ucha = she whispers in my ear

Polish often does this instead of using a possessive adjective like mojego ucha.

Using mojego here would usually sound unnecessary unless you wanted special emphasis.