Penso que aquesta idea és important.

In Catalan, the subject pronoun is often left out because the verb ending already tells you who the subject is.

• penso = I think
• The -o ending shows it is first person singular

So:

• Penso que aquesta idea és important. = I think that this idea is important.
• Jo penso que aquesta idea és important. also works, but jo is usually added only for emphasis, contrast, or clarity.

For example:

• Jo penso això, però ell no. = I think that, but he doesn’t.

Here, que means that and introduces a subordinate clause.

• Penso = I think
• que aquesta idea és important = that this idea is important

So the structure is:

• Penso que... = I think that...

This is very common in Catalan, just as in English.

Other examples:

• Crec que tens raó. = I think / believe that you are right.
• Diu que vindrà. = He/She says that he/she will come.

In casual English, that is often omitted, but in Catalan que is normally kept.

Because idea is a feminine noun in Catalan.

Catalan demonstratives must agree in gender and number with the noun:

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

Since idea is feminine singular, you need:

• aquesta idea = this idea

Compare:

• aquest llibre = this book
• aquesta casa = this house

In Catalan, nouns have grammatical gender, and idea is feminine, just as it is in several other Romance languages.

You can see that from the words around it:

• aquesta idea → feminine demonstrative
• If there were an article, it would be la idea

So:

• la idea
• aquesta idea
• una idea

A useful habit is to learn nouns together with an article or determiner:

• la idea, not just idea

És is the 3rd person singular form of the verb ser (to be), meaning is.

The accent helps distinguish it from es, which is a very common unstressed pronoun in Catalan.

• és = is
• es = reflexive/passive pronoun, like himself/herself/itself or part of passive structures

Examples:

• Aquesta idea és important. = This idea is important.
• Es diu Marta. = Her name is Marta. / She is called Marta.

So the accent is important because it marks a different word.

Some Catalan adjectives change form for gender, but others do not.

Important is an adjective that has the same form for masculine and feminine singular:

• un tema important = an important topic
• una idea important = an important idea

What changes is the plural:

• temes importants
• idees importants

So in this sentence, important does agree with idea, but the feminine singular happens to look the same as the masculine singular.

Yes. That would change the meaning slightly.

• Aquesta idea és important. = This idea is important.
• Penso que aquesta idea és important. = I think that this idea is important.

The first is a direct statement. The second makes it clear that this is your opinion or judgment.

So Penso que adds a personal viewpoint.

Yes, you could, and it would sound very natural.

• Penso que... = I think that...
• Crec que... = I think / I believe that...

In many contexts, both are possible. The difference is often small:

• penso can feel a bit more like I think / in my opinion
• crec can feel a bit more like I believe

But in everyday use, they often overlap.

So both of these are good:

• Penso que aquesta idea és important.
• Crec que aquesta idea és important.

This is the normal Catalan order:

• subject: aquesta idea
• verb: és
• adjective/complement: important

So the pattern is:

• This idea
  • is
    • important

Catalan word order is often similar to English in simple statements.

You usually would not move the adjective before the noun here:

• aquesta idea important does not mean this idea is important
• it would mean something more like this important idea, which is a different structure

So if you want to say is important, you need the verb:

• Aquesta idea és important

A practical pronunciation guide is:

• aquesta ≈ uh-KES-tuh

Some notes:

• the stress is on ques: aQUES ta
• the qu before e sounds like a hard k
• the final a is a neutral, unstressed vowel in many accents

So:

• aquesta idea ≈ uh-KES-tuh ee-DE-uh

Exact pronunciation varies somewhat by dialect, but that guide is good enough for learners.

A rough pronunciation guide is:

• Penso que aquesta idea és important
• ≈ PEN-so kuh uh-KES-tuh ee-DE-uh es im-por-TANT

A few key points:

• Penso: stress on the first syllable
• que is usually a short unstressed word
• aquesta: stress on ques
• idea: usually stressed on de
• és: short but clearly accented
• important: stress on the final syllable

If you want to sound natural, try saying it in two chunks:

• Penso que
• aquesta idea és important

Yes.

• Penso que aquesta idea és molt important. = I think that this idea is very important.

Here:

• molt = very
• important = important

So:

• important = important
• molt important = very important

This is a very common pattern in Catalan.

Yes, that would be wrong in standard Catalan.

With demonstratives like aquest / aquesta, you normally do not use the article el / la at the same time in this kind of phrase.

So you say:

• aquesta idea = this idea

Not:

• la aquesta idea

This is similar to English, where you say this idea, not the this idea.