La sangre es roja.

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

Grammatical gender in Spanish does not always match any logical or biological idea, especially for things and abstract nouns. You simply learn each noun with its gender:

• la sangre
• la leche
• la piel

Also, nouns ending in -e can be either masculine or feminine, so you cannot always guess the gender from the ending alone.

In Spanish, it is very common to use the definite article with nouns when talking about things in general.

So Spanish often says:

• La sangre es roja.
• La nieve es blanca.
• El oro es caro.

In English, you often leave the article out in these general statements, but in Spanish the article sounds natural and is usually required.

Because ser is used for general truths, essential characteristics, and definitions, while estar is used more for states, conditions, or locations.

Here, being red is presented as a general characteristic of blood, so Spanish uses ser:

• La sangre es roja.

If you used está, it would sound unusual here, because you would be treating red like a temporary condition rather than a general fact.

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

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

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

So:

• el coche rojo
• la sangre roja

Even after ser, the adjective still agrees with the noun:

• La sangre es roja.

Normally, no. In standard Spanish, that sounds unnatural.

When making a general statement about a noun like blood, Spanish normally uses the definite article:

• La sangre es roja.

Leaving out the article would not sound like normal everyday Spanish here.

A simple guide:

• sangre ≈ SAHN-greh
• roja ≈ ROH-kha or ROH-ha

A few details:

• In sangre, the g is a hard g sound, like in go, because it comes before r.
• In roja, the j is the strong Spanish sound heard in Spain, like a throaty h/ch sound.
• The stress is:
  • SAN-gre
  • RO-ja

Because in this sentence, roja is not directly attached to the noun inside the noun phrase. It is part of the structure noun + ser + adjective:

• La sangre = the subject
• es = the verb
• roja = the adjective describing the subject

This is the normal pattern in Spanish, just like in English:

• Blood is red
• La sangre es roja

If you wanted the adjective directly next to the noun, you would make a noun phrase instead:

• la sangre roja = the red blood

That is a different structure from La sangre es roja.

Yes, but it means something different.

• La sangre es roja = a full sentence
• la sangre roja = a noun phrase, meaning the red blood

So la sangre roja is not a complete statement by itself. It would usually appear inside a larger sentence, for example:

• Vi la sangre roja en la camisa.

Yes. It is the normal, neutral word order in Spanish:

• subject + verb + adjective

You could rearrange the words for emphasis in very special contexts, but for a learner, La sangre es roja is the standard and natural form.

Normally, sangre is an uncountable noun, so it is usually kept in the singular.

But if you ever had a plural noun, the adjective would still have to agree in number as well as gender. For example:

• Las rosas son rojas.

So the same agreement rule applies:

• singular noun → singular adjective
• plural noun → plural adjective

With sangre, though, the singular is the normal form here.