Όταν ακούω βροντή, κλείνω αμέσως το παράθυρο.

Greek often leaves out subject pronouns when they are clear from the verb ending.

• ακούω = I hear
• κλείνω = I close

So εγώ is unnecessary unless you want extra emphasis, such as Εγώ κλείνω το παράθυρο, όχι εσύ.

Όταν means when or whenever.

In this sentence, it has a habitual meaning:

• Όταν ακούω βροντή... = Whenever I hear thunder...

So it is not talking about one single event only; it describes what the speaker usually does.

Because the sentence describes a general habit or repeated action.

• ακούω = I hear
• κλείνω = I close

Together they mean something like:

• Whenever I hear thunder, I immediately close the window.

Greek uses the present tense for this kind of repeated, regular situation, just as English often does.

Yes, if you want to talk about one future occasion rather than a habit.

For example:

• Όταν ακούσω βροντή, θα κλείσω αμέσως το παράθυρο.

That means:

• When I hear thunder, I will immediately close the window.

So the difference is:

• Όταν ακούω... κλείνω... = a usual reaction / habit
• Όταν ακούσω... θα κλείσω... = one future event

It can relate to both ideas, depending on context, but here it clearly means hear.

• ακούω βροντή = I hear thunder

If you want to say listen, Greek often still uses ακούω, for example:

• Ακούω μουσική = I listen to music

So Greek uses ακούω more broadly than English sometimes does.

Because it is being used in a general sense: thunder as a phenomenon.

• ακούω βροντή = I hear thunder

This sounds natural in Greek. If you used the article, it would sound more specific, such as referring to a particular thunder sound already known from context.

Also:

• μια βροντή would mean a thunderclap or a clap of thunder

Greek often uses the singular for natural phenomena or general concepts, just as English can say I hear thunder.

So:

• βροντή = thunder in a general sense

If you use the plural βροντές, it suggests separate rumbles or thunderclaps:

• Ακούω βροντές = I hear thunderclaps / rumbles of thunder

Το παράθυρο means the window.

It is in the accusative because it is the direct object of κλείνω:

• κλείνω το παράθυρο = I close the window

The article is το because παράθυρο is a neuter singular noun.

Αμέσως means immediately or right away.

In this sentence:

• κλείνω αμέσως το παράθυρο = I immediately close the window

Its position is natural, but Greek word order is fairly flexible. You could also hear:

• κλείνω το παράθυρο αμέσως

The meaning stays very similar, though the emphasis may shift slightly.

No, Greek word order is more flexible than English word order.

The given sentence is very natural:

• Όταν ακούω βροντή, κλείνω αμέσως το παράθυρο.

But you could also say:

• Κλείνω αμέσως το παράθυρο όταν ακούω βροντή.

That still means the same thing. The original version is especially clear because it presents the condition/situation first and then the reaction.

Because Όταν ακούω βροντή is a subordinate clause placed before the main clause.

So the sentence is structured like this:

• Όταν ακούω βροντή, = subordinate time clause
• κλείνω αμέσως το παράθυρο. = main clause

This comma is normal and helps show the pause between the two parts.

A simple pronunciation guide is:

• Όταν ≈ O-tan
• ακούω ≈ a-KOO-o
• βροντή ≈ vro-DEE
• κλείνω ≈ KLEE-no
• αμέσως ≈ a-ME-sos
• το παράθυρο ≈ to pa-RA-thi-ro

The stress falls where the accent mark shows it:

• Όταν
• ακούω
• βροντή
• κλείνω
• αμέσως
• παράθυρο

Yes. In fact, whenever often captures the meaning very well in this sentence.

• Όταν ακούω βροντή, κλείνω αμέσως το παράθυρο.
• Whenever I hear thunder, I immediately close the window.

English often uses when for this too, but whenever makes the habitual sense especially clear.