Hem pensat en una sorpresa petita, però la festa serà gran.

Hem pensat is the present perfect in Catalan.

• hem = we have
• pensat = past participle of pensar = thought

So the structure is:

• haver
  • past participle

In this sentence, hem pensat means something like we have thought of / we came up with.

Catalan uses this tense very naturally for a completed action that is relevant now. Here, the idea of the surprise has already been thought of, and it matters to what comes next.

Because the verb here is pensar en, which means to think about or to think of.

So:

• pensar en alguna cosa = to think of/about something

In this sentence:

• Hem pensat en una sorpresa petita = We’ve thought of a small surprise

This is something English speakers often have to memorize, because prepositions do not always match exactly between English and Catalan.

Examples:

• Penso en tu = I’m thinking about you
• Hem pensat en un pla = We’ve thought of a plan

Una is the feminine singular form of the indefinite article, meaning a or an.

It matches sorpresa, which is a feminine singular noun.

So:

• un = masculine singular
• una = feminine singular

Examples:

• un regal = a gift
• una sorpresa = a surprise

Because sorpresa is feminine, Catalan uses una.

Because adjectives in Catalan usually agree with the noun in gender and number.

Here the noun is:

• sorpresa = feminine singular

So the adjective must also be feminine singular:

• petit = masculine singular
• petita = feminine singular
• petits = masculine plural
• petites = feminine plural

So:

• una sorpresa petita = correct
• una sorpresa petit = incorrect

In Catalan, adjectives often come after the noun.

So:

• una sorpresa petita literally follows the order a surprise small

This is very normal in Catalan.

That said, some adjectives can come before the noun for style or emphasis, but the basic and most common position is after the noun. For a learner, it is safest to think:

• noun + adjective is very common in Catalan

Examples:

• una casa gran = a big house
• un cotxe nou = a new car

Però means but.

The accent mark shows the correct stress and also helps distinguish it from other similar-looking forms. In pronunciation, the stress falls on the second syllable:

• pe-RÒ

So the sentence is connecting two contrasting ideas:

• a small surprise
• but the party will be big

That contrast is exactly what però is doing.

Because però introduces a contrast, and Catalan normally uses a comma before it in sentences like this.

So:

• Hem pensat en una sorpresa petita, però la festa serà gran.

This works much like English:

• We’ve thought of a small surprise, but the party will be big.

It separates the two coordinated clauses clearly.

Catalan often uses the definite article where English might or might not use the.

Here, la festa means the party. It sounds like a specific party already known from the context.

• la = feminine singular definite article
• festa = party

Because festa is feminine singular, the article is la.

Examples:

• la casa = the house
• la sorpresa = the surprise

Serà is the future tense of ser for he/she/it will be.

Here it refers to la festa, which is singular:

• la festa serà gran = the party will be big

So:

• serà = will be

This is the simple future form, built from the infinitive ser plus the future ending.

A few future forms of ser are:

• seré = I will be
• seràs = you will be
• serà = he/she/it will be
• serem = we will be
• sereu = you all will be
• seran = they will be

Gran is a very common way to say big in Catalan.

In this sentence:

• la festa serà gran = the party will be big

With events like festa, gran sounds very natural.

Also, gran can sometimes mean not only big/large but also great/important, depending on context. Here, because it contrasts with petita, the meaning is clearly about size or scale.

A useful thing to remember:

• petit / petita = small
• gran = big

In the singular, gran stays the same for both masculine and feminine.

So you get:

• un dia gran
• una festa gran

In the plural, it becomes:

• grans

Examples:

• uns edificis grans = some big buildings
• unes festes grans = some big parties

So gran is a little easier than adjectives like petit/petita, because the singular form does not change by gender.

Normally, no—not with the same meaning.

If you want to say we thought of a small surprise, Catalan normally uses:

• pensar en

So:

• Hem pensat en una sorpresa petita = natural

Without en, it would sound wrong or at least unnatural in this context.

This is one of those verb + preposition combinations that should be learned as a unit:

• pensar en = think of/about

A learner-friendly pronunciation guide would be something like:

em pen-SAT en OO-nə sur-PRAY-zə pə-TEE-tə, pe-RÒ lə FES-tə sə-RA gran

A few helpful notes:

• Hem is often pronounced with a silent h
• però is stressed on the last syllable
• serà is also stressed on the last syllable
• unstressed vowels may sound softer than in English, especially in many eastern varieties of Catalan

You do not need perfect regional pronunciation right away, but it helps to notice where the stress falls:

• pensat
• però
• serà

Yes, depending on context and variety, Catalan can use other past forms, but hem pensat is very natural here.

For many learners, the key point is:

• hem pensat = we have thought of / we’ve come up with

In some contexts, you may also see other past expressions, but this sentence is a straightforward and idiomatic way to say it.

So if your goal is to understand and use this sentence naturally, hem pensat is the form to focus on.