La montura negra parece ligera.

Because montura is a feminine noun in Spanish. Nouns have grammatical gender, and montura takes feminine words with it:

• la montura
• una montura
• montura negra
• montura ligera

This is why the article is la, not el.

They end in -a because they must agree with montura, which is feminine singular.

In Spanish, adjectives usually match the noun in:

• gender: masculine or feminine
• number: singular or plural

So:

• masculine singular: negro, ligero
• feminine singular: negra, ligera
• masculine plural: negros, ligeros
• feminine plural: negras, ligeras

Since montura is feminine singular, the sentence uses negra and ligera.

In Spanish, descriptive adjectives usually come after the noun.

So la montura negra is the normal word order for the black frame / mounting / saddle, depending on context.

Putting the adjective before the noun is sometimes possible, but it usually sounds:

• more literary
• more emotional
• more emphatic
• or it may slightly change the nuance

For example, la negra montura sounds much less neutral than la montura negra.

Parece is the third-person singular form of the verb parecer.

The subject of the sentence is la montura negra, which is singular, so the verb must also be singular:

• La montura negra parece ligera.
• Las monturas negras parecen ligeras.

So parece is used because the black frame/mounting is one thing, not several.

Because parece means seems or appears, while es means is.

That difference matters:

• La montura negra es ligera = it definitely is light
• La montura negra parece ligera = it seems light, or looks light

So the sentence does not fully state lightness as a certain fact. It presents it as an impression.

In this sentence, parece applies only to ligera.

So the structure is basically:

• La montura negra = the black frame/mounting
• parece ligera = seems light

That means:

• black is presented as a fact
• light is presented as an appearance or impression

If you wanted both qualities to fall under seems, you would normally say something like:

• La montura parece negra y ligera.

Yes, in a normal full sentence like this, la is the natural choice.

Spanish usually uses an article with a specific countable noun in this kind of statement:

• La montura negra parece ligera.

Without the article:

• Montura negra parece ligera

that sounds unnatural in standard Spanish, except perhaps in notes, labels, headlines, or very elliptical contexts.

Montura has several possible meanings depending on context. Common ones include:

• a frame or mounting
• the frame of glasses
• a saddle
• a support or fitted structure for holding something

So the exact meaning depends on the situation. Grammatically, though, the sentence works the same way in all cases.

Everything would need to agree in the plural:

• Las monturas negras parecen ligeras.

Changes:

• la → las
• montura → monturas
• negra → negras
• parece → parecen
• ligera → ligeras

This is a good example of how many parts of a Spanish sentence change together.

In standard Spanish from Spain, ligera is pronounced approximately lee-HE-ra.

A few useful points:

• the g before e sounds like the strong Spanish j sound
• the stress falls on ge: li-GE-ra
• the r here is the single tapped r, not the strong rolled one

So an English speaker should not pronounce the g like in get. It is much closer to the Spanish sound in jefe or gente.