Si no trobes la clau, pots trucar a la meva germana.

Yes. In Catalan:

• si = if
• sí = yes

So in this sentence, Si no trobes la clau... means If you don’t find the key... The accent changes the meaning.

Both are present indicative, second person singular forms, because the sentence is addressing you informally (tu).

• trobes comes from trobar = to find
• pots comes from poder = to be able to / can

So:

• trobes = you find
• pots = you can

Catalan often uses the present tense in everyday conditional sentences like this one.

Because Catalan often omits subject pronouns when the verb ending already shows who the subject is.

• trobes already tells you it means you find
• pots already tells you it means you can

You could add tu for emphasis, but it is not necessary:

• Si tu no trobes la clau, pots trucar a la meva germana.

That sounds more emphatic, like If you can’t find the key...

That is the normal way to make a sentence negative in Catalan: no goes before the conjugated verb.

• trobes = you find
• no trobes = you don’t find

So Si no trobes la clau literally follows the Catalan pattern If not you-find the key, which is the normal equivalent of If you don’t find the key.

Because la clau means the key, referring to a specific key.

Catalan uses the definite article very much like English here:

• clau = key
• la clau = the key

If the meaning is the key, you need the article.

Catalan usually uses the definite article + possessive + noun pattern:

• la meva germana = my sister
• el meu cotxe = my car
• la teva casa = your house

This is one of the big differences from English. English says just my sister, but Catalan normally says the my sister structurally, although in natural English translation we simply say my sister.

Because in Catalan, when trucar means to call someone, it normally takes the preposition a before the person:

• trucar a algú = to call someone

So:

• trucar a la meva germana = to call my sister

This is just how the verb works in Catalan. English does not use a preposition here, but Catalan does.

Not always. Trucar can mean:

• to call / phone
• to knock / ring, depending on context

For example:

• Trucar a la meva germana = to call my sister
• Trucar a la porta = to knock on the door
• Trucar al timbre = to ring the doorbell

In your sentence, because the object is a person, the meaning is clearly to call / phone.

Most naturally, pots means can here.

• pots trucar = you can call

Depending on context, English might also translate it as you could call, because it is offering a possible action or suggestion. But grammatically, pots is the present tense of poder, so the most direct meaning is you can.

Because Catalan commonly uses the present indicative for a real, possible condition.

• Si no trobes la clau, pots trucar...
• If you don’t find the key, you can call...

This is very natural in Catalan, just as it is in English.

You could also use a future idea in some contexts, but this version sounds straightforward and idiomatic: it describes a real possibility and a practical option.

Yes. It is informal singular, because it uses the tu forms:

• trobes
• pots

A formal singular version would be:

• Si no troba la clau, pot trucar a la meva germana.

That uses vostè-style verb forms.

If you were talking to more than one person informally, you would say:

• Si no trobeu la clau, podeu trucar a la meva germana.

Yes. You can put the main clause first:

• Pots trucar a la meva germana si no trobes la clau.

This means the same thing: You can call my sister if you can’t find the key.

The version with Si... first puts the condition up front, which is very common.

Because the sentence begins with a conditional clause:

• Si no trobes la clau, = If you don’t find the key,
• pots trucar a la meva germana. = you can call my sister.

Writing a comma after an initial if clause is standard and helps readability. If you put the condition second, you normally would not need that comma:

• Pots trucar a la meva germana si no trobes la clau.

A rough pronunciation guide is:

• Si ≈ see
• no ≈ noh
• trobes ≈ TROH-bəs
• la clau ≈ luh KLAU (with clau sounding roughly like clow/cow)
• pots ≈ pots
• trucar ≈ troo-CAR
• a la meva germana ≈ uh luh MAY-vuh jer-MAH-nuh

A more Catalan-like pronunciation in Central Catalan would reduce some unstressed vowels, especially the a and e sounds. Also:

• the g in germana sounds like the s in measure
• the r is tapped or rolled, not the English r

So even if your pronunciation is not perfect yet, those are the main sounds to notice.