Kiedy słyszę grzmot, od razu zamykam okno.

Polish usually drops subject pronouns when they are clear from the verb ending.

• słyszę = I hear
• zamykam = I close / I am closing

So ja is not necessary here. You could say Ja kiedy słyszę grzmot, od razu zamykam okno, but that would usually add emphasis, like I do this.

Here kiedy means when.

In this sentence it introduces a time clause:

• Kiedy słyszę grzmot = When I hear thunder / When I hear a thunderclap

It is followed by a comma before the main clause:

• Kiedy słyszę grzmot, od razu zamykam okno.

Because Kiedy słyszę grzmot is a subordinate clause, and in Polish subordinate clauses are normally separated from the main clause by a comma.

So the structure is:

• Kiedy słyszę grzmot = subordinate clause
• od razu zamykam okno = main clause

This comma is standard and important in Polish writing.

Słyszę is the 1st person singular present tense form of słyszeć (to hear).

So:

• słyszę = I hear
• słyszysz = you hear
• słyszy = he/she/it hears

In this sentence, it matches the understood subject I.

Because grzmot is the direct object of słyszę, so it is in the accusative case. But for many masculine inanimate nouns in Polish, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: grzmot
• accusative: grzmot

That is why the form does not change.

Grzmot usually means a thunderclap, peal of thunder, or crash of thunder—something countable that you hear.

So this sentence feels a bit like:

• When I hear a thunderclap, I immediately close the window.

If you wanted the more general idea of thunder as a weather phenomenon, Polish often uses grzmoty in the plural or other expressions depending on context. But grzmot is very natural here because it refers to a sound event.

Od razu means immediately, right away, or straight away.

So:

• od razu zamykam okno = I immediately close the window

It is a very common Polish expression.

Examples:

• Od razu wracam. = I’m coming right back.
• Zrób to od razu. = Do it right away.

Zamykam is the imperfective verb, while zamknę is perfective.

Here zamykam is natural because the sentence describes a repeated or habitual action:

• Whenever I hear thunder, I close the window right away.

That is a general pattern, not a one-time future event.

Compare:

• Kiedy słyszę grzmot, od razu zamykam okno.
  = When I hear thunder, I immediately close the window.
  (habit / repeated behavior)

• Kiedy usłyszę grzmot, od razu zamknę okno.
  = When I hear thunder, I’ll close the window right away.
  (future, one completed action)

Okno is the direct object of zamykam, so it is in the accusative case. But okno is a neuter noun, and in the singular its accusative form is the same as the nominative form.

So:

• nominative: okno
• accusative: okno

That is why it stays okno.

Most naturally, it describes a habit or a repeated reaction.

• Kiedy słyszę grzmot, od razu zamykam okno.
• Whenever I hear thunder, I close the window immediately.

Polish present tense is often used for regular behavior, just like English present simple.

In the right context, it could also sound like a vivid description of what happens, but the default reading is habitual.

Yes. Gdy can also mean when.

So you could say:

• Gdy słyszę grzmot, od razu zamykam okno.

This is very similar in meaning. In many contexts:

• kiedy is a bit more common and neutral in everyday speech
• gdy can sound slightly more formal or literary, though it is still normal Polish

Yes, Polish word order is fairly flexible, though some versions sound more neutral than others.

The original sentence is very natural:

• Kiedy słyszę grzmot, od razu zamykam okno.

You could also say:

• Od razu zamykam okno, kiedy słyszę grzmot.

This is still correct, but it shifts the focus a little. The original version is usually the most straightforward if you want to emphasize the when clause first.

A rough pronunciation guide:

• Kiedy ≈ KYEN-dih
• słyszę ≈ SWIH-shen
• grzmot ≈ GZH-mot
• od razu ≈ od RAH-zoo
• zamykam ≈ za-MIH-kam
• okno ≈ OK-no

A few useful notes:

• ł sounds like English w
• sz sounds like sh
• rz here sounds like zh as in measure
• y is not exactly like English i; it is a distinct Polish vowel

So the whole sentence is approximately:

KYEN-dih SWIH-shen GZH-mot, od RAH-zoo za-MIH-kam OK-no.