El prensatelas baja sobre la placa de aguja.

Because in Spanish, some singular nouns do end in -s. Prensatelas is one of those. You can tell it is singular here because it goes with el and the singular verb baja.

This is similar to other Spanish nouns such as el paraguas or el lunes, which also end in -s but can be singular.

In fact, the plural is usually the same form: el prensatelas / los prensatelas.

It is a compound noun, the kind of noun Spanish often forms from a verb plus a noun. These compounds are very common for tools, devices, or parts.

Words of this type are often:

• masculine
• treated as a single fixed word
• sometimes unchanged in the plural

So it is best to learn el prensatelas as one complete vocabulary item, not by trying to translate each piece literally.

Here, baja is a verb. It is the third person singular present of bajar.

So in this sentence, baja means something like goes down, lowers, or comes down, depending on the context.

It is not the adjective baja meaning low or short. You can tell it is a verb because it comes after the subject el prensatelas and is followed by a prepositional phrase: sobre la placa de aguja.

Spanish often uses the simple present to describe:

• how something works
• what a machine part does
• general facts
• steps in instructions or manuals

So baja sounds natural in technical or descriptive language. English sometimes does the same, but Spanish uses this present tense very comfortably in these contexts.

Sobre is a preposition. In this sentence it means something like over, onto, or on top of, depending on how the action is being understood.

Because the verb bajar involves downward movement, sobre here suggests that the presser foot moves down so that it ends up over or onto the needle plate.

So sobre is marking the position or destination of that downward movement.

Because placa de aguja works like a technical name for a machine part. In Spanish, when de + noun is used to identify the type or function of something, the article is often left out.

This is very common in fixed expressions such as:

• cepillo de dientes
• máquina de coser
• caja de herramientas

Here, de aguja tells you what kind of plate it is. It is not mainly talking about a specific needle that owns the plate. That is why placa de aguja sounds natural.

If you said placa de la aguja, it would sound more like you are referring to the plate of a particular needle, not the standard name of the part.

The article agrees with the main noun, not with the noun after de.

In la placa de aguja:

• the main noun is placa
• de aguja only adds more information

So the article must match placa, which is feminine singular: la placa.

The gender of aguja does not control the article here.

Yes, but the normal, neutral order is:

El prensatelas + baja + sobre la placa de aguja

That is the most straightforward order for a descriptive sentence.

You could change the order for emphasis, for example:

Sobre la placa de aguja baja el prensatelas

But that sounds more marked or stylistic. For most learners, the original order is the one to remember and use first.