Kiedy widzę błyskawicę za oknem, zamykam je od razu.

Here kiedy literally means when, but in this kind of sentence it often has the sense of whenever.

Because both verbs are in the present tense, the sentence sounds like a general, repeated situation:

Kiedy widzę błyskawicę za oknem, zamykam je od razu.
= Whenever I see lightning outside the window, I close it immediately.

So in practice, when and whenever are both good ways to understand it.

They are in the present tense because Polish often uses the present for habitual actions or general truths.

So this sentence is not mainly about one specific moment. It describes what the speaker usually does:

• widzę = I see
• zamykam = I close

In English, we might say:

• When I see lightning outside the window, I close it immediately
• or more naturally, Whenever I see lightning outside the window, I close it immediately

Both are 1st person singular: I see and I close.

• widzę comes from widzieć = to see
• zamykam comes from zamykać = to close

So the subject is understood as I, even though Polish does not need to say ja here.

A full version with the pronoun would be:

Kiedy ja widzę błyskawicę za oknem, ja zamykam je od razu.

But that sounds unnecessary and unnatural in normal Polish, because the verb endings already show who is acting.

Because błyskawicę is the accusative singular form of błyskawica.

The verb widzieć normally takes a direct object in the accusative case:

• widzieć kogo? co? = to see whom? what?

So:

• nominative: błyskawica = lightning
• accusative: błyskawicę

That is why Polish says:

widzę błyskawicę = I see lightning

This is very common with feminine nouns ending in -a:

• książka → książkę
• kobieta → kobietę
• błyskawica → błyskawicę

Yes. Błyskawica is a normal word for lightning, especially the visible flash.

A related word is piorun, which often means lightning bolt or thunderbolt. In some contexts, the two can feel close, but they are not always exactly the same.

Very roughly:

• błyskawica = the flash of lightning
• piorun = a lightning strike / thunderbolt

In this sentence, błyskawica sounds perfectly natural.

Because the preposition za can take different cases depending on meaning.

Here it expresses location, not movement, so it uses the instrumental:

• za oknem = behind the window / outside the window

The noun okno becomes oknem in the instrumental singular.

Compare:

• za oknem = location, already there
• za okno = movement to a place behind/beyond the window

So in this sentence, the lightning is outside, not moving somewhere, which is why oknem is used.

Literally, za oknem is behind the window. But in everyday English, the most natural translation here is outside the window or outside.

In weather-related contexts, za oknem is very commonly used to mean what is happening outside, as seen from indoors.

So here the idea is:

I see lightning outside the window
not that the lightning is somehow physically hidden behind the glass.

Je refers to okno = window.

Since okno is a neuter singular noun, the pronoun used for it is je.

So:

• okno = window
• zamykam je = I close it

Even though je comes after oknem, it does not refer to lightning. It refers back to the window.

Because je is the correct pronoun for a neuter singular direct object.

The noun okno is neuter, so when it is replaced by it as the object of zamykam, the form is je.

So the logic is:

• zamykam okno = I close the window
• zamykam je = I close it

This is one of those places where it helps to remember the gender of the noun:

• ten for masculine nouns
• ta for feminine nouns
• to for neuter nouns like okno

Because zamykam is imperfective, and it fits a repeated/habitual action.

Polish often uses the imperfective present when talking about what someone regularly does:

• zamykać = to be closing / to close habitually
• zamknąć = to close once, to complete the action

So:

• zamykam je od razu = I close it immediately as a usual reaction
• zamknę je would normally mean I will close it in the future, as one completed action

That is why zamykam is the natural choice here.

Od razu means immediately, right away, or at once.

So:

zamykam je od razu = I close it immediately

It is a very common Polish expression. You can use it in many everyday situations:

• Zrób to od razu = Do it right away
• Oddzwonię od razu = I’ll call back immediately

Because Kiedy widzę błyskawicę za oknem is a subordinate clause, and in Polish such clauses are normally separated by a comma.

So the structure is:

• Kiedy widzę błyskawicę za oknem = subordinate time clause
• zamykam je od razu = main clause

In Polish punctuation, this comma is required.

Yes. Polish word order is quite flexible, although the original version is very natural.

The given sentence is neutral and clear:

Kiedy widzę błyskawicę za oknem, zamykam je od razu.

You could also say:

Od razu zamykam je, kiedy widzę błyskawicę za oknem.

But that shifts the emphasis a bit toward od razu. The original sentence sounds like the most standard way to express the idea.

Sometimes Polish can omit an object if it is obvious from context, but here je helps make the sentence clear and natural.

So:

• zamykam je od razu = clearly I close it immediately
• zamykam od razu could work only if everyone already knows what is being closed

In this sentence, keeping je is the better choice.

It most naturally describes a repeated habit or usual reaction.

That comes from the combination of:

• kiedy
• present tense verbs
• imperfective zamykam

So the feeling is:

Whenever I see lightning outside the window, I immediately close the window.

If you wanted one specific past event, Polish would usually use past forms instead.