La casa és molt gran.

Because casa is a feminine singular noun in Catalan, so it takes the feminine singular definite article la.

• la casa = the house
• el is the masculine singular article, so it would be used with masculine nouns, not casa.

This is something you usually have to learn together with the noun: casa is feminine.

És means is.

It is the 3rd person singular present tense of the verb ser (to be):

• jo sóc = I am
• tu ets = you are
• ell/ella és = he/she is

So in La casa és molt gran, és is the verb that links the house with the description very big.

The accent in és is important for both spelling and pronunciation.

It helps distinguish és (is) from es, which can be a pronoun in Catalan.

It also shows the stressed vowel quality. In standard Catalan, és has a clear stressed e sound.

So you should write:

• és = is

not:

• es when you mean the verb

Because molt means very, and in Catalan, as in English, very goes before the adjective it modifies.

• molt gran = very big
• molt petit = very small
• molt interessant = very interesting

So the structure is:

• noun + ser
  • molt
    • adjective

Here:

• La casa = the house
• és = is
• molt gran = very big

Because gran is an adjective that has the same form for masculine and feminine singular.

So you get:

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

In the plural, it changes to:

• cotxes grans
• cases grans

So gran works perfectly with casa, even though casa is feminine.

Because this sentence describes a basic characteristic of the house: its size.

In Catalan, ser is generally used for:

• identity
• essential characteristics
• general descriptions

And estar is more often used for:

• location
• temporary states
• conditions

A house being big is considered an inherent description, so Catalan uses ser:

• La casa és molt gran.

Not està molt gran.

A simple learner-friendly pronunciation would be:

lah KAH-zuh ez molt grahn

A few useful notes:

• la is short and light
• casa sounds roughly like KAH-zuh
• és sounds like ez
• molt has a clear l
• gran ends with a strong n, and the r is Catalan/Spanish-style, not an English r

If you want a more standard Central Catalan approximation, it is often close to:

lə KAH-zə ez molt gran

Yes, in this sentence the word order is very similar to English:

• La casa = the house
• és = is
• molt gran = very big

So the pattern is:

subject + verb + adverb + adjective

That makes this sentence quite straightforward for an English speaker.

Normally, no.

If you mean the house, Catalan usually needs the article:

• La casa és molt gran.

Without the article, casa would not normally sound natural as the subject in an ordinary sentence like this.

Catalan uses definite articles a lot, and in this case la is required.

Not always. Gran can have several related meanings depending on context, such as:

• big / large
• great
• sometimes older / grown-up

But in La casa és molt gran, with house, the meaning is clearly big or large.

So here there is no ambiguity:

• La casa és molt gran = The house is very big.