Según la crítica, el mayor defecto del sistema es la falta de justicia.

Según means “according to”. It introduces the source of an opinion, judgment, or information.

In this sentence, Según la crítica = “According to the critics / According to the criticism”.

Basic pattern:

• según + noun/pronoun
  
  • Según Juan, eso es falso. → According to Juan, that’s false.
  • Según la ley, no se puede hacer. → According to the law, you can’t do that.

You normally put según at the beginning of the clause, but it can also appear after the main statement:

• El mayor defecto del sistema es la falta de justicia, según la crítica.
  (This is also correct and sounds natural.)

Spanish uses la crítica in a special way:

1. la crítica (singular, feminine) can mean:

   • “criticism” as an abstract noun
   • “the critics” as a collective group (film critics, literary critics, etc.)

   In Según la crítica, it usually means “according to the critics (as a group)”.

2. el crítico / la crítica (with a person as reference):

   • el crítico = the (male) critic
   • la crítica = the (female) critic

3. las críticas (plural) usually means:

   • “the criticisms” or “the reviews” (individual opinions or reviews)

So:

• Según la crítica = According to the critics / the critical establishment
• Según las críticas = According to the (individual) reviews
• Según el crítico = According to the (male) critic

Both are grammatically correct, but they differ in nuance:

• el mayor defecto

  • Literally: “the greater / greatest defect”
  • Usual translation: “the biggest / main / most serious flaw”
  • Sounds more formal and abstract, often used for importance or seriousness.

• el defecto más grande

  • Literally: “the biggest defect”
  • Focuses more on size/degree in a neutral way.
  • Still understandable in this context, but el mayor defecto is more idiomatic when talking about the main problem with a system, idea, plan, etc.

So el mayor defecto del sistema naturally means “the greatest flaw of the system / the system’s main defect.”

Spanish requires a contraction of de + el (preposition + masculine singular article):

• de + el = del
  
  • del sistema = of the system

You must use del, not de el, when el is the masculine singular article:

• ✅ del libro (of the book)
• ❌ de el libro (incorrect)

There is no contraction with other articles:

• de la ciudad
• de los alumnos
• de las personas

Also, there is no contraction with the pronoun él (with accent):

• de él = of him (no contraction)

Literally, falta de justicia means “lack of justice” or “absence of justice.”

• falta here is a noun meaning “lack / shortage / absence.”
• falta de + noun is a very common pattern to express “lack of X”:
  • falta de dinero = lack of money
  • falta de respeto = lack of respect
  • falta de información = lack of information

In this sentence:

• la falta de justicia = the lack of justice (treated as a specific issue)

Don’t confuse this with faltar (the verb “to be lacking / to be missing”), e.g.:

• Falta justicia. = Justice is lacking. / There is a lack of justice.

In la falta de justicia, the article la belongs to falta, not to justicia:

• la falta = the lack
• de justicia = of justice (justice in general)

When a noun is abstract and general after de, Spanish often omits the article:

• amor de madre (mother’s love)
• falta de tiempo (lack of time)
• uso de tecnología (use of technology)

If you say de la justicia, you usually refer to a specific, previously mentioned kind of justice, or something like “the justice system”:

• la falta de la justicia en este país
  = the lack of (the) justice in this country
  (sounds more like “of the justice system in this country”)

Here, the idea is general: “a lack of justice (as a principle)”, so justicia appears without article.

Yes, you can, but there’s a nuance:

• es la falta de justicia

  • “is the lack of justice”
  • Sounds more definite and focused: this specific main problem is the lack of justice.
  • Fits well after el mayor defecto del sistema (“the main defect of the system”).

• es falta de justicia

  • “is a lack of justice / is lack of justice”
  • Sounds more like a description or classification, less like identifying a single, main defect.
  • Often used in moral judgments:
    
    • Eso es falta de respeto. = That’s a lack of respect.
    • Esto es falta de justicia. = This is a lack of justice.

In this particular sentence, es la falta de justicia matches better because it is defining what the main defect is.

Mayor is one of a small group of comparative/superlative adjectives that are invariable in gender:

• mayor (older / greater)
• menor (younger / lesser)
• mejor (better)
• peor (worse)

They do not change for masculine/feminine:

• el mayor defecto
• la mayor ventaja
• el mejor libro
• la mejor idea

They do change for number:

• los mayores problemas
• las mayores ventajas

So mayor never becomes mayora.

Es is the present indicative of ser, used to state facts, definitions, and generally accepted opinions:

• …el mayor defecto del sistema es la falta de justicia.
  = “…the greatest defect of the system is the lack of justice.”

Using sea (present subjunctive) would need a different structure, usually after a verb or expression that triggers the subjunctive, for example:

• Puede que el mayor defecto del sistema sea la falta de justicia.
  = It may be that the greatest defect of the system is the lack of justice.

In the original sentence, the idea is presenting the criticism as a statement of fact (according to the critics), so es is the natural and correct choice.