Si no trobo la clau, em quedo preocupada.

In Catalan, si without an accent means if.

• si = if
• sí = yes

So in Si no trobo la clau..., si introduces a condition: If I don’t find the key...

Catalan often leaves out subject pronouns when the verb already makes the subject clear.

• trobo = I find / I am finding
• em quedo = I get / I remain

Because the verb endings already show the person, jo is usually unnecessary unless you want emphasis or contrast.

So:

• Si no trobo la clau... = If I don’t find the key...
• Si jo no trobo la clau... would be more emphatic, like If I don’t find the key...

Trobo is the 1st person singular present tense of trobar.

• trobar = to find
• trobo = I find

In this sentence, no trobo la clau means I don’t find the key or more naturally in English, I can’t find the key.

Catalan often uses the simple present where English might use either:

• I don’t find
• I’m not finding
• I can’t find

depending on context.

Catalan usually uses the definite article more often than English does.

• la clau = the key

Here it refers to a specific key, so la clau is the normal choice.

This is very common in Catalan:

• Tanco la porta = I close the door
• Busco el mòbil = I’m looking for my phone / the phone

Even when English might sometimes omit the, Catalan often keeps the article.

Em is the 1st person singular reflexive/object pronoun, meaning myself / me, depending on the verb.

Here it goes with quedar-se, which is a very common Catalan verb structure.

• quedar = to remain / to stay / to be left
• quedar-se = to become / to remain / to stay, depending on context

So:

• em quedo preocupada literally looks like I stay myself worried
• but naturally it means I get worried or I remain worried

You should think of quedar-se as a fixed verbal expression in many contexts.

In Catalan, quedar-se + adjective often means to become or to end up in a state.

Examples:

• quedar-se sorprès/sorpresa = to become surprised
• quedar-se sol/sola = to be left alone
• quedar-se preocupat/preocupada = to get worried

So em quedo preocupada does not literally mean just I stay worried in the English sense. In context, it usually means:

• I get worried
• I become worried

That is a very natural Catalan way to express a change of state.

Because adjectives in Catalan usually agree with the gender and number of the person they describe.

• preocupat = worried, masculine singular
• preocupada = worried, feminine singular

So em quedo preocupada tells you the speaker is female.

If the speaker were male, it would be:

• Si no trobo la clau, em quedo preocupat.

Grammatically, both verbs are in the present tense:

• trobo = I find
• em quedo = I get/stay

But with si clauses, Catalan often uses the present tense to talk about a real future possibility, just like English can do:

• If I don’t find the key, I get worried
• more naturally: If I don’t find the key, I’ll get worried

So even though the verb form is present, the meaning can refer to what happens in the future or in general situations.

No is the normal negation word in Catalan.

• trobo = I find
• no trobo = I don’t find

So:

• Si no trobo la clau... = If I don’t find the key...

This works very much like Spanish and unlike English, which needs do:

• Catalan: no trobo
• English: I do not find

Yes — that is actually a very natural way to understand it in English.

Catalan no trobo literally means I don’t find, but in context English often prefers I can’t find.

So possible natural translations are:

• If I don’t find the key, I get worried.
• If I can’t find the key, I get worried.
• If I can’t find the key, I start to worry.

The exact English version depends on the context, but the Catalan is perfectly normal.

Yes, but the meaning changes.

• estic preocupada = I am worried
  This describes your state.
• em quedo preocupada = I get worried / I become worried
  This emphasizes the change into that state.

So:

• Si no trobo la clau, estic preocupada = If I don’t find the key, I am worried.
• Si no trobo la clau, em quedo preocupada = If I don’t find the key, I get worried.

The sentence you have focuses more on becoming worried.

The structure is:

• Si
  • negative verb clause
• main clause

So:

• Si no trobo la clau = If I don’t find the key
• em quedo preocupada = I get worried

This is a very normal pattern in Catalan for conditional sentences.

You could also reverse the order:

• Em quedo preocupada si no trobo la clau.

That still means the same thing: I get worried if I don’t find the key.

Yes. Clau is feminine, which is why it takes la:

• la clau = the key

Some common examples:

• la clau de casa = the house key
• les claus = the keys

So the article matches the noun:

• singular feminine: la clau
• plural feminine: les claus