Es mejor que el resumen sea breve y claro.

Because sea is the present subjunctive form of ser.

The pattern Es mejor que... often expresses a recommendation, preference, or judgment rather than a plain fact, and that usually triggers the subjunctive in the next clause:

• Es mejor que el resumen sea breve y claro.
• Es importante que llegues pronto.
• Es necesario que estudien más.

If you used es here, it would sound ungrammatical in standard Spanish.

The trigger is Es mejor que...

Expressions like these commonly require the subjunctive because they introduce something desired, advisable, or evaluated:

• Es mejor que... = It is better that...
• Es bueno que... = It is good that...
• Es importante que... = It is important that...
• Es posible que... = It is possible that...

So in this sentence, the speaker is not simply stating a fact about the summary; they are saying what would be preferable.

Que links the two parts of the sentence:

• Es mejor = It is better
• que el resumen sea breve y claro = that the summary be brief and clear

So que here means that.

In English, that is often optional:

• It’s better that the summary be brief and clear
• It’s better for the summary to be brief and clear

In Spanish, que is required in this structure.

Spanish often uses the definite article more than English does.

Here, el resumen means the summary, referring to a specific summary or to the summary in a general instructional sense.

Saying just resumen would not work in this sentence. Spanish normally needs an article or another determiner before the noun:

• el resumen = the summary
• un resumen = a summary
• mi resumen = my summary

In Spanish, most descriptive adjectives normally come after the noun:

• un resumen breve
• una explicación clara
• un texto interesante

So el resumen sea breve y claro follows the usual pattern.

With ser, these adjectives are part of the predicate, not directly attached before the noun. Compare:

• el resumen breve = the brief summary
• el resumen sea breve = the summary should be brief

Both are normal, but they are different structures.

Because they agree with resumen, which is a singular masculine noun.

• el resumen → masculine singular
• therefore: breve and claro must also be singular, and claro must be masculine

Examples:

• el resumen sea breve y claro
• la explicación sea breve y clara
• los resúmenes sean breves y claros

Notice that breve has the same form for masculine and feminine singular, but it changes in the plural:

• breve
• breves

Because breve and claro here describe the essential qualities the summary should have, not a temporary state.

• ser breve = to be brief
• ser claro = to be clear

These are seen as characteristics of the summary itself.

Using estar would sound unnatural here. Estar is usually for states, conditions, or results, not for the kind of defining quality meant in this sentence.

So:

• sea breve y claro = correct
• esté breve y claro = not natural here

Sea is the first-person singular and third-person singular form of the present subjunctive of ser.

Full present subjunctive of ser:

• yo sea
• tú seas
• él / ella / usted sea
• nosotros / nosotras seamos
• vosotros / vosotras seáis
• ellos / ellas / ustedes sean

In this sentence, it matches el resumen, which is third-person singular:

• el resumen sea

Sometimes estar claro is possible, but it means something slightly different.

• ser claro = to be clear by nature / in style / in wording
• estar claro = to be clear, obvious, evident

So:

• Es mejor que el resumen sea claro.
  = It’s better for the summary to be clearly written / clear in style.

• Está claro que...
  = It is clear that...

For breve y claro describing how a summary should be written, ser is the natural choice.

Yes. That is a very close, grammatical English translation.

However, in everyday English, people often prefer something slightly more natural, such as:

• It’s better for the summary to be brief and clear.
• The summary should be brief and clear.

The Spanish structure is still worth learning because it is very common:

• Es mejor que + subjunctive

Yes. A few common alternatives are:

• Es mejor que el resumen sea breve y claro.
• Conviene que el resumen sea breve y claro.
  = It is advisable for the summary to be brief and clear.
• El resumen debe ser breve y claro.
  = The summary must/should be brief and clear.
• Es preferible que el resumen sea breve y claro.
  = It is preferable that the summary be brief and clear.

They are all similar, but es mejor que sounds like a recommendation, while debe ser sounds stronger and more direct.

You can use an infinitive when the subject is general or the same as the main clause, but here the sentence has a specific subject: el resumen.

So Spanish uses:

• Es mejor que el resumen sea breve y claro.

If you say:

• Es mejor ser breve y claro,
  that means It is better to be brief and clear, in a general sense, without specifically naming the summary as the subject.

So both are grammatical, but they do not mean exactly the same thing.

In Spain, a typical pronunciation would be roughly:

Es mejor que el resumen sea breve y claro.

A few points:

• j in mejor is a strong throaty sound, like the ch in Scottish loch
• que sounds like keh
• c in claro is a normal k sound
• in most of Spain, z in resumen is not present, so nothing special there
• v in breve is pronounced much like b in Spanish

A rough English-style guide: es meh-HOR keh el reh-SOO-men SEH-ah BREH-beh ee KLA-ro

Not perfect, but useful as a starting point.