Si hi ha una tempesta forta, primer veiem el llampec i després sentim el tro.

In Catalan, hi ha is the usual way to say there is or there are.

• ha comes from the verb haver
• hi is a little word that is part of this existential expression

So:

• hi ha una tempesta = there is a storm
• hi ha dues persones = there are two people

Even though English changes between there is and there are, Catalan normally just uses hi ha for both.

Si means if.

So:

• Si hi ha una tempesta forta... = If there is a strong storm...

A very common thing for learners to notice is this:

• si = if
• sí = yes

The accent changes the meaning.

In Catalan, adjectives usually come after the noun.

So:

• tempesta forta = strong storm
• literally: storm strong

This is the normal word order in Catalan. English often puts adjectives before the noun, but Catalan usually places them after it.

Also, forta agrees with tempesta:

• tempesta is feminine singular
• so the adjective is forta, not fort

Because tempesta is a feminine noun.

In Catalan, adjectives must agree with the noun in:

• gender
• number

So:

• un vent fort = a strong wind
• una tempesta forta = a strong storm

Here:

• tempesta = feminine singular
• forta = feminine singular form of fort

Primer here means first or first of all.

In this sentence, it works as an adverb marking the sequence of events:

• primer veiem el llampec = first we see the lightning flash

You could sometimes see longer forms like primerament, but primer is very common and natural in everyday Catalan.

Catalan often leaves out subject pronouns because the verb ending already shows who the subject is.

• veiem = we see
• sentim = we hear

So Catalan does not need to say nosaltres unless you want emphasis or contrast.

Compare:

• veiem = we see
• nosaltres veiem = we see / we are the ones who see

The shorter version is the normal one.

Veiem is the 1st person plural present indicative of veure (to see).

So:

• 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

In the sentence, veiem is used because the statement is a general fact: when there is a strong storm, we see the lightning first.

Sentim is the 1st person plural present indicative of sentir.

So:

• jo sento or sometimes sent depending on variety/context
• tu sents
• ell/ella sent
• nosaltres sentim
• vosaltres sentiu
• ells/elles senten

Here it means we hear.

The present tense is used because the sentence expresses a general truth or scientific fact.

It is not about one specific storm in the past or future. It is saying what normally happens:

• first we see the lightning
• then we hear the thunder

Catalan, like English, often uses the present tense for this kind of general statement.

Catalan often uses the definite article where English might not focus on it as much.

Here, el llampec and el tro refer to the lightning flash and the thunder associated with the storm. Using el sounds natural and specific in context.

So:

• el llampec = the lightning flash
• el tro = the thunder

Also remember:

• llampec is a masculine noun
• tro is also masculine

That is why both use el.

Llampec usually means a flash of lightning.

In weather contexts, Catalan can use related words, but llampec is a very natural choice when talking about the visible flash in a storm.

So in this sentence:

• veiem el llampec = we see the lightning flash
• sentim el tro = we hear the thunder

This makes the cause-and-effect sequence very clear.

Després means afterwards, then, or later.

It comes before the verb here because it is organizing the sequence of events:

• primer veiem el llampec
• i després sentim el tro

This is a very normal word order in Catalan.

Sometimes yes, but they are not always exactly the same.

• després focuses on afterward / later
• llavors often means then / at that moment / in that case, depending on context

In this sentence, després is the most straightforward choice because it describes the order in time: first one thing happens, then the other.

Because i simply joins the two coordinated parts of the sentence:

• primer veiem el llampec
• i després sentim el tro

In normal Catalan punctuation, you usually do not put a comma before i unless there is some special reason, such as emphasis or a more complex structure.

The comma that does appear is after the si clause:

• Si hi ha una tempesta forta, ...

That comma separates the condition from the main part of the sentence.

The ll in Catalan is traditionally a separate sound, similar to a palatal ly sound, though pronunciation can vary by region and by speaker.

A rough guide for learners:

• llampec sounds approximately like lyam-PEC

Also:

• the stress is on the last syllable: llamPÈC

If you are a beginner, the most important thing is to recognize the word and pronounce it consistently; regional pronunciation differences are normal in Catalan.

Yes, but the meaning changes slightly.

• Si hi ha una tempesta forta... = If there is a strong storm...
• Quan hi ha una tempesta forta... = When there is a strong storm...

In this sentence, both can make sense because it describes what happens in that situation. But:

• si presents it as a condition
• quan presents it as something that happens whenever that situation occurs

So si is fine, and quan would also be possible in a slightly different framing.