Learn Hardcore Spanish (Latin America): La esfera azul brilla sobre la mesa.

In Spanish every noun has a gender (masculine or feminine) and number (singular or plural). Esfera is a feminine, singular noun, so it requires the feminine singular definite article La (“the”). If you were talking about more than one sphere, you’d say Las esferas.

What’s the difference between esfera and bola?

Both words can translate as “ball” or “sphere” in English, but:

Esfera refers specifically to a perfect geometric sphere (math or formal contexts).
Bola is more general: a round object, a ball you play with, a scoop of ice cream, etc.
In your sentence the emphasis is on the shape (a perfect sphere), so esfera is more precise.

Why doesn’t azul change ending to azula to agree with esfera?

Spanish adjectives must agree in gender and number with the noun, but azul is one of several color adjectives that are invariable for gender. It only changes in number:

Singular: azul
Plural: azules
So with a feminine singular noun you still say esfera azul, not azula.

Why does the adjective azul come after the noun esfera instead of before it?

The default word order in Spanish is noun + adjective. Placing azul after esfera simply follows that norm. You could put azul before for stylistic or poetic reasons—La azul esfera brilla…—but it sounds more formal or poetic. In everyday speech and writing, colors typically follow the noun.

What form of the verb brillar is brilla, and why is it in that form?

Brilla is the third person singular present indicative of brillar (“to shine”). We use it because our subject is la esfera (she/it):

Yo brillo (I shine)
Tú brillas (you shine)
Él/Ella/La esfera brilla (he/she/it shines)

What nuance does brillar have compared to iluminar?

Brillar is intransitive: the subject itself emits or reflects light (“to shine,” “to sparkle”).
Iluminar is transitive: it means “to light up” or “to illuminate” something else.
So you say La esfera brilla when the sphere itself is glowing or reflecting light; you’d say La lámpara ilumina la mesa if a lamp is lighting up the table.

Why is sobre used instead of en, and could you use encima de instead?

En simply means “in” or “on.” Using en la mesa would still be correct: La esfera azul brilla en la mesa.
Sobre means “on top of” or “above.” It adds a slight sense of “above the surface.”
Encima de is very close in meaning to sobre, and you could definitely say encima de la mesa:
“La esfera azul brilla encima de la mesa.”
All three are grammatically correct; the choice depends on how precise or formal you want to be.

Why do we use the definite article in sobre la mesa instead of omitting it?

Spanish often uses definite articles where English drops them. Here, la mesa refers to a specific table (perhaps the one you and the speaker both know about). If you meant “on a table” (any table), you’d use the indefinite article: sobre una mesa. Using la signals that both speaker and listener understand which table is meant.