Quan veig un llampec, tanco la finestra.

Quan means when. It introduces a time clause: Quan veig un llampec = When I see a lightning flash.

Putting it first is very natural in Catalan, just like in English:

• Quan veig un llampec, tanco la finestra.
• When I see lightning, I close the window.

You could also put the main clause first:

• Tanco la finestra quan veig un llampec.

Both are correct.

Veig is the 1st person singular form of the verb veure, which means to see.

So:

• veig = I see

A few present-tense forms of veure are:

• jo veig = I see
• tu veus = you see
• ell/ella veu = he/she sees
• nosaltres veiem = we see
• vosaltres veieu = you all see
• ells/elles veuen = they see

This is a common irregular verb, so veig does not look exactly like the infinitive veure.

Llampec usually means flash of lightning.

So un llampec is:

• a flash of lightning
• sometimes more simply lightning, depending on context

This sentence suggests a single visible flash:

• Quan veig un llampec... = When I see a flash of lightning...

Catalan also has related weather words such as:

• llamp = lightning bolt / thunderbolt
• tro = thunder

Catalan usually uses an article where English often can go without one.

Here, un llampec means a flash of lightning, referring to one instance.

Compare:

• veig un llampec = I see a flash of lightning
• veig llampecs = I see lightning flashes / flashes

Using un makes it sound like a specific occurrence each time the situation happens.

Tanco means I close. It comes from the verb tancar, meaning to close.

So:

• tanco la finestra = I close the window

Some present-tense forms of tancar:

• jo tanco = I close
• tu tanques = you close
• ell/ella tanca = he/she closes
• nosaltres tanquem = we close
• vosaltres tanqueu = you all close
• ells/elles tanquen = they close

Catalan often leaves out subject pronouns because the verb ending usually makes the subject clear.

So instead of saying:

• Jo veig un llampec, jo tanco la finestra

Catalan normally says:

• Quan veig un llampec, tanco la finestra.

The forms veig and tanco already show that the subject is jo = I.

You can add jo for emphasis, contrast, or clarity:

• Quan veig un llampec, jo tanco la finestra.

But in a neutral sentence, leaving it out is more natural.

Catalan often uses the present tense for habitual actions or general truths, just like English does.

So this sentence means something like:

• Whenever I see a flash of lightning, I close the window.

It is not necessarily happening right now. It can describe a regular habit or normal reaction.

If you wanted a past meaning, the verbs would change:

• Quan vaig veure un llampec, vaig tancar la finestra.
• When I saw a flash of lightning, I closed the window.

The comma separates the time clause from the main clause:

• Quan veig un llampec = subordinate clause
• tanco la finestra = main clause

This is very similar to English:

• When I see a flash of lightning, I close the window.

If the main clause comes first, the comma is often omitted:

• Tanco la finestra quan veig un llampec.

Here, quan can be understood as when or whenever, depending on context.

In a sentence with the present tense like this, it often has a habitual meaning:

• Quan veig un llampec, tanco la finestra.
• Whenever I see a flash of lightning, I close the window.

So even though quan literally means when, the full sentence often implies whenever.

La finestra means the window. The definite article is used because the speaker probably means a specific, known window, such as the one in the room or house.

Compare:

• tanco la finestra = I close the window
• tanco una finestra = I close a window

In everyday situations, the window is often the natural choice.

Yes, it can suggest an immediate reaction, especially from context, but its most basic meaning is simply:

• When / Whenever I see a flash of lightning, I close the window.

If you want to emphasize as soon as, Catalan can use other expressions, for example:

• Tan bon punt veig un llampec, tanco la finestra.
• As soon as I see a flash of lightning, I close the window.

So the original sentence can imply quick action, but it does not explicitly focus on immediacy.

The sentence is natural as written, but Catalan word order is somewhat flexible.

Natural versions include:

• Quan veig un llampec, tanco la finestra.
• Tanco la finestra quan veig un llampec.

Both mean the same thing.

However, the original order is especially common when you want to set the time or condition first:

• When I see a flash of lightning, I close the window.

Usually no, not in the same way.

• quan = when
• si = if

So:

• Quan veig un llampec, tanco la finestra. = When/Whenever I see a flash of lightning, I close the window.
• Si veig un llampec, tanco la finestra. = If I see a flash of lightning, I close the window.

The version with si sounds more conditional and hypothetical. The version with quan sounds more like a regular or expected situation.