La mordaza de carpintero se cierra.

De carpintero means something like for carpentry, used by a carpenter, or carpenter’s in a general sense.

• mordaza de carpintero = a carpenter’s clamp / a woodworking clamp
• mordaza del carpintero = the carpenter’s clamp, meaning a specific clamp belonging to a specific carpenter

So de carpintero is describing the type of clamp, not ownership by one particular carpenter.

Grammatically, carpintero is still a noun meaning carpenter, but in the phrase de carpintero it works like a descriptive label.

Spanish often uses this structure:

• mesa de madera = wooden table
• zapatos de deporte = sports shoes
• mordaza de carpintero = carpenter’s clamp / woodworking clamp

So instead of turning carpintero into a separate adjective, Spanish uses de + noun.

Spanish usually uses the definite article more often than English. La mordaza means the clamp.

Even when English might say just clamp in a technical label, Spanish often keeps the article in a full sentence:

• La puerta se abre.
• La caja se cierra.
• La mordaza de carpintero se cierra.

So la is completely normal here.

The verb here is cerrarse, which can mean to close in an intransitive sense: the thing itself closes.

Compare:

• Cerrar algo = to close something
  
  • Cierro la puerta. = I close the door.
• Cerrarse = to close
  
  • La puerta se cierra. = The door closes.

So La mordaza de carpintero se cierra means the clamp closes, not that someone is closing it in this sentence.

It is formally reflexive because it uses se, but in meaning it often works more like an intransitive or middle-voice construction than a truly reflexive one.

A truly reflexive idea would be something like He washes himself. Here, the clamp is not really doing something to itself in a conscious way. Spanish simply uses se very often with verbs like:

• abrirse = to open
• cerrarse = to close
• romperse = to break

So for a learner, the safest understanding is:

• cerrar = to close something
• cerrarse = to close

Sometimes se + verb can have a passive-like meaning, but in this sentence the most natural reading is simply closes.

Compare:

• La puerta se cierra. = The door closes.
• Se cierran las tiendas a las ocho. = The shops are closed at eight / Shops close at eight.

So context decides the best translation. For La mordaza de carpintero se cierra, the straightforward reading is that the clamp closes.

Because Spanish normally does not need subject pronouns unless there is a special reason to use them.

The verb form cierra already tells you it is:

• él / ella / usted cierra

And the noun phrase La mordaza de carpintero is already the subject, so adding ella would be unnecessary.

It is the present simple: cerrarse → se cierra.

This tense can mean different things depending on context, for example:

• a general fact
• a habitual action
• what something does by design
• a description happening now

So here it could mean:

• The clamp closes
• The clamp is closing in some contexts
• The clamp closes shut / can be closed depending on the situation

Because the subject is singular: La mordaza.

• La mordaza ... se cierra = singular
• Las mordazas ... se cierran = plural

The phrase de carpintero does not affect the number of the verb. The verb agrees with mordaza, which is the main noun.

Then you would use cerrar without the reflexive se, because now the carpenter is acting on the clamp:

• El carpintero cierra la mordaza.

That means The carpenter closes the clamp.

So the contrast is:

• La mordaza de carpintero se cierra. = The carpenter’s clamp closes.
• El carpintero cierra la mordaza. = The carpenter closes the clamp.

Not necessarily. Tool vocabulary can vary by region and by trade. In Spain, people may also use terms such as sargento for a type of clamp. However, mordaza de carpintero is understandable as a descriptive phrase for a carpenter’s clamp.

So for learning purposes, the grammar of the sentence is perfectly useful even if a different tool word might also be common in real-life workshops.