Les despeses són mensuals.

Les is the feminine plural definite article in Catalan, meaning the.

In this sentence, despeses is a plural feminine noun, so the article has to match:

• la despesa = the expense
• les despeses = the expenses

A quick comparison:

• el = masculine singular
• la = feminine singular
• els = masculine plural
• les = feminine plural

Despeses is plural. The singular form is despesa.

So:

• la despesa = the expense
• les despeses = the expenses

The ending -es is a very common plural ending for feminine nouns in Catalan.

Because the subject is plural.

• és = is (3rd person singular)
• són = are (3rd person plural)

Since les despeses means the expenses, Catalan uses the plural verb:

• La despesa és mensual. = The expense is monthly.
• Les despeses són mensuals. = The expenses are monthly.

Són comes from the verb ser, which means to be.

Here is the present tense of ser:

• jo sóc = I am
• tu ets = you are
• ell/ella és = he/she is
• nosaltres som = we are
• vosaltres sou = you all are
• ells/elles són = they are

In your sentence, les despeses is a plural noun, so Catalan uses són.

Because it has to agree with the plural noun despeses.

In Catalan, adjectives usually agree with the noun in number and often in gender too.

Here:

• mensual = monthly (singular)
• mensuals = monthly (plural)

So:

• La despesa és mensual.
• Les despeses són mensuals.

Notice that mensual does not change for masculine vs. feminine here; it only changes for singular vs. plural:

• pagament mensual = monthly payment
• quota mensual = monthly fee
• pagaments mensuals = monthly payments
• despeses mensuals = monthly expenses

Because in Catalan, adjectives often come after the noun.

So despeses mensuals is the normal order for monthly expenses.

This is very common in Catalan:

• una casa gran = a big house
• un problema difícil = a difficult problem
• despeses mensuals = monthly expenses

English often puts adjectives before the noun, but Catalan usually places them after it.

In this sentence, mensuals means monthly.

It describes the expenses as having a monthly frequency or recurring each month.

Depending on context, English might translate the whole sentence in different natural ways, such as:

• The expenses are monthly.
• These are monthly expenses.
• The expenses occur every month.

But grammatically, mensuals is an adjective meaning monthly.

A simple learner-friendly pronunciation guide is:

les des-PE-zes son men-su-ALS

A few helpful notes:

• despeses has the stress on PE
• són has a clear stressed o
• mensuals has the stress on the last part: -als

A rough IPA version is:

[ləz dəsˈpɛ.zəs son mən.suˈals]

Pronunciation varies somewhat by dialect, but this is a good general guide.

Because that is the correct standard spelling of the word.

Són is written with an accent, and you should learn it as a fixed form of the verb ser.

The accent also helps show the stressed vowel. In normal writing, you should write:

• són = are

not

• son

In standard written Catalan, no. The correct form is són.

You may sometimes see missing accents in casual typing, text messages, or when someone is using a keyboard without easy accent input, but for correct Catalan you should write:

Les despeses són mensuals.

Yes. Despesa / despeses is a very common and standard Catalan word for expense / expenses.

Examples:

• Les despeses de la casa = household expenses
• Despeses mensuals = monthly expenses
• Reduir les despeses = to reduce expenses

So this sentence sounds natural and normal Catalan.

No, that would not be natural here.

Catalan uses ser in this sentence because mensuals is describing a characteristic or classification of the expenses: they are monthly.

So the correct sentence is:

Les despeses són mensuals.

Using estar would not fit this meaning.

The singular version would be:

La despesa és mensual.

You can see how everything changes to singular:

• les → la
• despeses → despesa
• són → és
• mensuals → mensual

This is a very useful pattern to notice in Catalan.