La candidatura i la carta de presentació són importants.

Because Catalan normally repeats the definite article before each noun when both nouns are specific and definite.

So:

• la candidatura i la carta de presentació = the application and the cover letter

Leaving out the second la would usually sound incomplete or less natural in a normal sentence.

Because the subject is plural in meaning.

Even though la candidatura is singular and la carta de presentació is singular, they are joined by i (and), so together they form a plural subject:

• la candidatura
  • la carta de presentació → two things
• therefore són = are

Compare:

• La candidatura és important. = The application is important.
• La candidatura i la carta de presentació són importants. = The application and the cover letter are important.

Adjectives in Catalan agree with the noun or nouns they describe.

Since the subject is plural, the adjective must also be plural:

• singular: important
• plural: importants

So:

• és important = is important
• són importants = are important

Because Catalan forms the plural of many adjectives simply by adding -s.

For important:

• singular masculine/feminine: important
• plural masculine/feminine: importants

This adjective has the same form for masculine and feminine; only the number changes.

The article tells you: la is the feminine singular definite article.

So:

• la candidatura = feminine singular
• la carta = feminine singular

Also, nouns ending in -a are often feminine in Catalan, though that is a pattern, not an absolute rule.

It is a noun phrase made of:

• carta = letter
• de presentació = of presentation / introducing

Together, carta de presentació is the standard Catalan expression for cover letter.

The structure noun + de + noun is very common in Catalan.

Because presentació here is not referring to one specific presentation. It is part of a fixed expression that describes the type of letter.

So:

• carta de presentació = cover letter

Adding la would change the meaning and make it sound like the letter of the presentation, which is not what is meant here.

In són, the accent helps distinguish it from other words and marks the correct written form of the verb.

• són = they are
• és = is

For a learner, the important thing is simply to recognize són as the plural form of ser in the present tense.

It shows where the stress falls:

• pre-sen-ta-CIÓ

So the last syllable is stressed.

Accent marks in Catalan are important for both pronunciation and correct spelling.

i is pronounced like ee in English see.

So:

• la candidatura i la carta...

It simply means and.

Normally, no. Standard Catalan uses the plural here:

• són importants

That is because there are two coordinated nouns. Even if you think of them as one application package, the normal grammar is still plural in this kind of sentence.

Yes.

The structure is:

• Subject: La candidatura i la carta de presentació
• Verb: són
• Adjective: importants

This is a very standard pattern in Catalan:

• [noun phrase] + ser + adjective

For example:

• El currículum és important.
• Els documents són importants.

So your sentence follows a very common and natural structure.