Aquesta clàusula és important.

Because clàusula is a feminine singular noun, and the demonstrative has to agree with it.

• aquest = this (masculine singular)
• aquesta = this (feminine singular)
• aquests = these (masculine plural)
• aquestes = these (feminine plural)

So:

• aquest llibre = this book
• aquesta clàusula = this clause

Catalan agreement works more visibly than English here.

In this sentence, you can tell because it takes aquesta, the feminine form of this.

Also, nouns ending in -a are often feminine in Catalan, though not always. So clàusula follows a very common pattern.

You would normally say:

• la clàusula
• aquesta clàusula

If it were masculine, you would expect something like el or aquest instead.

És is the 3rd person singular of the verb ser = to be.

So:

• és = is

In this sentence:

• Aquesta clàusula és important = This clause is important

Catalan uses ser here because important is describing what the clause is, not where it is or how it is temporarily positioned.

Catalan, like Spanish, often distinguishes between ser and estar.

Here, és important is the natural choice because important is being presented as an essential quality or evaluation of the clause.

• és important = it is important

Using estar here would generally sound wrong or unnatural in standard usage.

A simple learner rule is:

• ser for identity, classification, general qualities
• estar for location, condition, temporary states

So important normally goes with ser in this kind of sentence.

Because important is a predicate adjective after the verb ser.

In English, we say:

• This clause is important

not

• This clause is an important

Catalan works the same way here:

• Aquesta clàusula és important

The adjective is simply describing the subject after és.

Yes, adjectives in Catalan often agree in gender and number with the noun they describe. But important is one of those adjectives that has the same form for masculine and feminine singular.

So you get:

• important = masculine singular
• important = feminine singular
• importants = plural

Examples:

• un punt important = an important point
• una clàusula important = an important clause
• punts importants = important points
• clàusules importants = important clauses

So in your sentence, the form important is correct for clàusula.

They help show stress and sometimes distinguish words.

clàusula

The accent in clàusula shows where the stress goes:

• CLÀU-su-la

Without the accent, the stress pattern would be less clear.

és

The accent in és helps distinguish it from es.

• és = is
• es = reflexive/object pronoun, like himself/herself/itself in some contexts, or part of pronominal verbs

So the accent is important both for pronunciation and meaning.

A rough pronunciation guide is:

• a-QUES-ta CLAU-su-la es im-por-TANT

A more IPA-style version is approximately:

• /əˈkɛs.tə ˈklaw.zu.lə ɛz im.purˈtan/

A few useful pronunciation notes:

• qu in aquesta sounds like a hard k
• au in clàusula sounds like ow in cow
• ll in clàusula here is just a normal l sound because the word has a single l, not ll
• the final -nt in important is pronounced

Exact pronunciation varies a bit by dialect.

Yes, but Aquesta clàusula és important is the most neutral and standard order.

This is the basic pattern:

• Subject + verb + adjective

You might also hear:

• És important aquesta clàusula

But that sounds more marked or emphatic, something like This clause is important with extra focus.

For a learner, the safest default is:

• Aquesta clàusula és important

That depends on the variety of Catalan.

In standard Central Catalan, aquesta is the usual form. In Valencian, esta is commonly used and perfectly normal.

So both may exist, but:

• aquesta = widely standard
• esta = common in Valencian usage

If you are learning general standard Catalan, aquesta clàusula és important is the best form to start with.