Prensamos el cuadernillo, a no ser que la tarlatana siga húmeda.

Because a no ser que means unless, and this expression normally requires the subjunctive in Spanish.

The idea after unless is treated as a possible exception, not as a confirmed fact. So la tarlatana siga húmeda means the tarlatan may still be wet, and that possibility is what stops the action in the main clause.

Also, siga is the present subjunctive form of seguir.

It is a fixed expression meaning unless.

A very literal breakdown would be something like if it is not the case that, but learners should really just remember it as one chunk: a no ser que = unless.

It is very commonly followed by the subjunctive:

• a no ser que venga
• a no ser que sea verdad
• a no ser que siga húmeda

Spanish often uses the present tense for procedures, instructions, scheduled next steps, and practical actions, especially in technical or workshop contexts.

So Prensamos el cuadernillo can naturally mean:

• We press the booklet
• We now press the booklet
• Next, we press the booklet

It does not have to mean a general habit. In this kind of context, the present tense often sounds more immediate and natural than the future.

For -ar verbs, the nosotros form of the present and the preterite is often identical:

• prensamos = we press / we are pressing / we press next
• prensamos = we pressed

So you use context to decide.

Here, the rest of the sentence points to a present or procedural meaning:

• a no ser que la tarlatana siga húmeda

Since siga is present subjunctive and the sentence sounds like a current instruction or step in a process, the natural reading is present, not past.

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

Prensamos already tells you the subject is nosotros/nosotras:

• prenso = I press
• prensas = you press
• prensamos = we press

So adding nosotros would usually only be for emphasis or contrast.

Because the adjective has to agree with the noun it describes.

Here the noun is la tarlatana, which is feminine singular, so the adjective must also be feminine singular:

• la tarlatana húmeda

If the noun were masculine, you would use húmedo instead.

Because seguir + adjective means to continue to be or to remain.

So:

• siga húmeda = is still wet / remains wet
• esté húmeda = is wet

Using seguir adds the idea that the fabric was already wet before and has not dried yet. That nuance is important here.

It is a technical word here.

In everyday Spanish, cuadernillo can mean a small notebook or booklet. But in printing or bookbinding, it often refers to a gathering or signature: a folded section of pages that forms part of a book.

So the best English equivalent depends on the wider context, but this is clearly specialist vocabulary.

Tarlatana is also a technical term. It refers to a loosely woven cotton fabric, often called tarlatan in English.

Depending on the field, it may be described more generally as a kind of gauze or muslin-like cloth, but in printing and bookbinding it has a specific technical use. That is why the sentence sounds specialized.

Yes, in most cases they mean the same thing: unless.

So you could also say:

• Prensamos el cuadernillo, a menos que la tarlatana siga húmeda.

Both expressions normally take the subjunctive. The difference is mostly one of style and preference, not basic meaning.

The comma separates the main statement from the exception clause.

Main clause:

• Prensamos el cuadernillo

Exception:

• a no ser que la tarlatana siga húmeda

This punctuation is very natural in Spanish when the unless phrase comes after the main clause. It helps show that the second part limits or qualifies the first.

Spanish uses definite articles more often than English, especially when talking about specific objects already known in the situation.

Here, the sentence is not talking about any booklet or any fabric in general. It refers to the particular booklet and the particular tarlatan involved in the process:

• el cuadernillo
• la tarlatana

In English, we might sometimes leave the article out in a procedural style, but Spanish normally keeps it.