La carta es roja.

La is the definite article, so it means the. It points to a specific letter/card/menu that both speaker and listener can identify.

Una would mean a / one, so Una carta es roja would mean A letter/card is red or One letter/card is red, depending on context.

So the difference is:

• La carta es roja. = The card/letter is red.
• Una carta es roja. = A card/letter is red.

Because carta is a feminine noun in Spanish, so it takes the feminine singular article la.

Spanish articles must agree with the noun:

• el = masculine singular
• la = feminine singular

So:

• la carta
• not el carta

A useful pattern is that many nouns ending in -a are feminine, and carta follows that pattern.

Because adjectives in Spanish usually agree with the noun they describe in gender and number.

Since carta is feminine singular, the adjective must also be feminine singular:

• masculine singular: rojo
• feminine singular: roja
• masculine plural: rojos
• feminine plural: rojas

So:

• el libro rojo
• la carta roja

And in this sentence:

• La carta es roja.

Es comes from ser, and está comes from estar. Both can mean is, but they are used differently.

Here, es roja describes a characteristic of the object: its color. For basic color identification, Spanish normally uses ser:

• La carta es roja.

That is the standard way to say that the card/letter is red.

Very generally:

• ser = what something is, its identity or characteristic
• estar = how something is, its condition or state

So for color, ser is the normal choice.

In normal Spanish, no. Singular count nouns usually need an article or another determiner.

So you would normally say:

• La carta es roja.
• Una carta es roja.
• Mi carta es roja.

But not usually:

• Carta es roja.

Spanish uses articles more often than English does in this kind of sentence.

In this sentence, roja is not directly attached to the noun inside the noun phrase. It comes after es, so it is a predicate adjective:

• La carta es roja. = The card is red.

If you put the adjective directly with the noun, you get:

• la carta roja = the red card

So compare:

• La carta es roja. → full sentence
• la carta roja → noun phrase

Also, in Spanish, adjectives often come after the noun when they directly modify it.

Yes, but La carta es roja is the most neutral and natural order.

Spanish word order is more flexible than English, but changing it usually adds emphasis or sounds literary.

Examples:

• La carta es roja. → normal
• Es roja la carta. → possible, but marked; sounds more emphatic
• Roja es la carta. → very marked or poetic

For a learner, the safest version is:

• La carta es roja.

A simple learner-friendly pronunciation is:

lah KAR-tah es ROH-hah

A few key points:

• c before a sounds like k
  
  • carta ≈ KAR-tah
• the r in carta is a single tap, not the strong rolled rr
• the j in roja sounds like a strong h sound in most of Latin America
  
  • roja ≈ ROH-hah

So the whole sentence is roughly:

lah KAR-tah es ROH-hah

Es is the third-person singular present form of the verb ser.

That means it is used with subjects like:

• él
• ella
• usted
• singular nouns such as la carta

Examples:

• Él es alto.
• Ella es médica.
• La carta es roja.

So in this sentence, es matches the singular subject la carta.

You make the noun, article, verb, and adjective agree in plural:

• La carta es roja. → singular
• Las cartas son rojas. → plural

Changes:

• la → las
• carta → cartas
• es → son
• roja → rojas

This is a very important Spanish pattern: article, noun, verb, and adjective all have to match correctly.