Encuadernamos los pliegos, a no ser que falte el cosido.

It is the 1st person plural present indicative of encuadernar.

So it means we bind or, depending on context, we are binding. In Spanish, the simple present often covers several English present-time meanings:

• Encuadernamos los pliegos cada mañana. = We bind the sheets every morning.
• Ahora encuadernamos los pliegos. = We’re binding the sheets now.

In this sentence, it most naturally sounds like a general procedure or habitual action.

Because Spanish usually drops subject pronouns when the verb ending already makes the subject clear.

• Encuadernamos already tells you the subject is we
• Adding nosotros is possible, but it is usually for emphasis, contrast, or clarity

So:

• Encuadernamos los pliegos... = neutral
• Nosotros encuadernamos los pliegos... = we bind the sheets (perhaps not someone else)

In general Spanish, pliego can mean a sheet, large sheet, or something folded. In printing and bookbinding, pliegos is a technical word and often refers to printed sheets or folded sections/signatures that will later become part of a book.

So this is not just random paper in the everyday sense. It sounds like bookbinding / printing vocabulary.

It means unless.

It is a fixed expression. Learners sometimes try to analyse it literally word by word, but it is best learned as a chunk:

• a no ser que = unless

A close equivalent is a menos que, which is also very common.

Examples:

• Iremos, a no ser que llueva. = We’ll go, unless it rains.
• Lo haré, a menos que cambies de idea. = I’ll do it, unless you change your mind.

Because a no ser que requires the subjunctive.

So after a no ser que, you normally use a subjunctive verb:

• a no ser que falte...
• not a no ser que falta...

Here, falte is the present subjunctive of faltar.

This is one of the most common things learners ask about: after expressions meaning unless, provided that, in case, so that, etc., Spanish often uses the subjunctive because the clause is seen as non-factual, conditional, or hypothetical.

Here faltar means to be missing or to be lacking.

So:

• que falte el cosido = that the stitching be missing / that the sewing be lacking

It does not mean to fail in the usual English sense here.

A useful way to think about faltar is:

• Falta algo = Something is missing
• Me falta dinero = I’m short of money / I lack money
• Falta una página = A page is missing

In this sentence, the thing that may be missing is el cosido.

Here it is functioning as a noun.

It comes from coser (to sew / stitch) and in this context el cosido means something like:

• the stitching
• the sewing
• the sewn binding / stitched part

This is common in Spanish: a past participle can be used as a noun, especially in technical language.

Compare:

• cosido as participle: un cuaderno cosido = a sewn notebook
• el cosido as noun: falta el cosido = the stitching is missing

In a bookbinding context, this sounds quite natural and technical.

Spanish often uses the definite article where English might not.

Here el cosido refers to a specific process, component, or stage in the binding process, so the article sounds natural.

Spanish often does this with abstract or technical nouns:

• falta el papel
• hay que revisar el pegado
• no está listo el cosido

In English, you might sometimes leave the article out depending on style, but Spanish often prefers it.

Because the main clause states a real action or general fact, so it uses the indicative:

• Encuadernamos los pliegos = We bind the sheets

The subordinate clause introduced by a no ser que expresses an exception or possibility, so it uses the subjunctive:

• a no ser que falte el cosido = unless the stitching is missing

So the mood choice reflects the function of each clause:

• indicative for the stated action
• subjunctive for the conditional exception

Yes. In most contexts, they mean essentially the same thing:

• Encuadernamos los pliegos, a no ser que falte el cosido.
• Encuadernamos los pliegos, a menos que falte el cosido.

Both mean We bind the sheets unless the stitching is missing.

If there is any difference, it is usually just stylistic or regional preference, not a big change in meaning.

Not if you want exactly the same meaning.

• a no ser que falte el cosido = unless the stitching is missing
• si no falta el cosido = if the stitching is not missing

These may be logically close in some situations, but they are not structured the same way and are not always interchangeable.

In your sentence, a no ser que is the more natural way to express unless.

Because the sentence has a main clause followed by an exception clause:

• Encuadernamos los pliegos,
• a no ser que falte el cosido.

That comma is very natural in writing. It helps separate the main statement from the limiting condition.

You may sometimes see punctuation vary in short sentences, but here the comma is standard and helpful.

It sounds technical, especially because of encuadernar, pliegos, and cosido in this combination.

A native speaker would likely understand that this belongs to the world of:

• printing
• publishing
• bookbinding
• paper production

So even if the grammar is completely normal, the vocabulary is somewhat specialised.

Most likely it sounds like a habitual or procedural present:

• We bind the sheets, unless the stitching is missing

That kind of sentence is very common in instructions, workflow descriptions, and professional explanations.

Spanish uses the present tense a lot for:

• routines
• instructions
• process descriptions
• general truths

So it does not necessarily mean right now at this exact moment.

Not exactly.

• costura usually means sewing, seam, or needlework in a broader sense
• cosido in this sentence sounds more like the stitched element / stitching process in a technical manufacturing or bookbinding context

So while both relate to sewing, el cosido is the better fit here because the sentence sounds like industry-specific language.