Las rodilleras nuevas son verdes y combinan con sus patines.

Rodilleras means knee pads, not knees.

• Rodilla = knee (the body part).
• Rodillera = one knee pad.
• Rodilleras = knee pads (the pair / in general).

So las rodilleras nuevas = the new knee pads.

In Spanish, every noun has a grammatical gender: masculine or feminine.

• Rodillera ends in -a, and it is feminine.
• The plural is rodilleras, still feminine.
• Feminine plural nouns take the article las.

So you must say las rodilleras, never los rodilleras.

Both word orders are possible, but they feel slightly different:

• Las rodilleras nuevas (adjective after the noun)

  • More neutral and common.
  • Just states that the knee pads are new, like giving a basic characteristic.

• Las nuevas rodilleras (adjective before the noun)

  • Often adds a bit of emphasis or contrast: these new ones, not the old ones.
  • Can sound a little more expressive.

Grammatically, both are correct; the difference is nuance, not meaning.

Both ser and estar can go with adjectives, but they express different ideas.

• Ser (+ color) = describing an essential or normal characteristic:

  • Las rodilleras nuevas son verdes. → Their color is green (that’s how they are).

• Estar (+ color) would suggest a temporary or unusual state, for example if they got stained:

  • Las rodilleras nuevas están verdes de pintura. → They are green (because of paint).

Here we’re just describing their normal color, so son is the right choice. Also, it’s plural (son) to agree with plural las rodilleras.

Adjectives in Spanish must agree in number (singular/plural) and often gender with the noun:

• Noun: las rodilleras → plural, feminine.
• Adjective: verde does not change for gender, but it does change for number:
  
  • singular: verde
  • plural: verdes

Because rodilleras is plural, the adjective must also be plural: verdes.

Combinar con means to match or to go well with in the sense of colors or style.

• Combinan con sus patines → (they) match his/her/their skates.

The subject is las rodilleras nuevas, so the verb is plural: combinan.

Common synonyms:

• Hacen juego con sus patines.
• Van bien con sus patines.

In Spanish, combinar in the sense of clothing or colors normally uses the preposition con:

• Esta camisa combina con esos pantalones.
• Las rodilleras combinan con sus patines.

Without con, combinar usually means something like to mix (for example, combining ingredients), which is a different meaning.

Sus is a possessive adjective that can mean:

• his skates
• her skates
• your skates (formal usted)
• their skates

Spanish sus is ambiguous in English; context usually tells you whose skates they are.

Also, sus is plural because it matches patines (plural noun):

• singular: su patín (his/her/its/your/their skate)
• plural: sus patines (his/her/its/your/their skates)

It does not change for masculine/feminine; only for singular/plural.

Spanish often uses the definite article for body-related items or personal items when it’s clear whose they are, instead of repeating the possessive.

For example:

• Me duele la cabeza. → My head hurts.
• Se puso las rodilleras. → He/She put (on) his/her knee pads.

In your sentence, the context may already make it clear whose knee pads they are, so las rodilleras nuevas sounds natural without sus. For patines, the speaker chooses to clarify possession explicitly with sus patines. Both patterns are normal in Spanish.

Patín / patines generally refers to skates (ice skates, roller skates, inline skates, etc.) in many Latin American varieties.

• Patín = one skate.
• Patines = a pair of skates / skates in general.

Since you normally use two skates, Spanish usually talks about them in the plural: sus patines = his/her/their skates.

Yes, that’s correct.

• Las rodilleras nuevas son verdes… (adjective after noun)
• Las nuevas rodilleras son verdes… (adjective before noun)

Both are grammatically fine and mean essentially the same thing. As mentioned before, las nuevas rodilleras can sound a bit more like these new knee pads (as opposed to other ones), but in everyday speech the difference is small.

Approximate pronunciation (Latin American):

• rodilleras → [roh-dee-YEH-ras]

  • ro like ro in rope (but shorter)
  • di like dee
  • lle pronounced like ye in yes (in most of Latin America)
  • stress on -ye-: ro-di-YE-ras

• patines → [pah-TEE-nes]

  • pa like pa in pasta
  • ti like tee
  • stress on -ti-: pa-TI-nes