El hilo encerado une los cuadernillos.

Because hilo is a masculine noun in Spanish, so it takes el in the singular: el hilo.

A good rule is that noun gender has to match the article:

• el hilo = the thread
• los hilos = the threads

This is grammatical gender, so it does not mean the object is “male.”

Encerado means waxed.

It comes from the verb encerar, which means to wax or to coat with wax. The form encerado is the past participle, and here it works like an adjective describing hilo:

• hilo = thread
• hilo encerado = waxed thread

Because hilo is masculine singular, the adjective also appears in the masculine singular form:

• encerado for a masculine singular noun
• encerada for a feminine singular noun
• encerados / enceradas for plurals

In Spanish, most descriptive adjectives usually come after the noun. So hilo encerado is the normal order.

This is one of the big differences from English:

• English: waxed thread
• Spanish: hilo encerado

Putting the adjective before the noun is sometimes possible in Spanish, but it often sounds literary, emphatic, or changes the nuance. In this sentence, hilo encerado is the natural everyday order.

Une is the third-person singular present tense of the verb unir, which means to join, to unite, or to connect.

So:

• yo uno = I join
• tú unes = you join
• él / ella / usted une = he / she / you joins

In El hilo encerado une los cuadernillos, the subject is el hilo encerado (singular), so the verb must also be singular: une.

Because the subject is singular: el hilo encerado.

Even though los cuadernillos is plural, that is the direct object, not the subject. The verb agrees with the subject, not with the object.

So:

• El hilo encerado une los cuadernillos. = The waxed thread joins the booklets/signatures.
• If the subject were plural, then you would use unen:
  • Los hilos encerados unen los cuadernillos.

Because los cuadernillos is a direct object referring to things, not people.

In Spanish, the personal a is normally used before a specific person (or sometimes a pet or personified being), for example:

• Veo a María.

But with things, you usually do not use a:

• Une los cuadernillos.
• Abre el libro.

So une los cuadernillos is correct without a.

Cuadernillos is the plural of cuadernillo.

In general Spanish, cuadernillo can mean a small notebook or booklet. But in contexts like bookbinding or printing, it often means signatures or gatherings: folded sections of pages that are sewn together to make a book.

So depending on context, los cuadernillos could be understood as:

• the small booklets
• the signatures / folded sections

The ending -illo is often a diminutive in Spanish. A diminutive can suggest small size, and sometimes familiarity or a technical subtype.

Here:

• cuaderno = notebook
• cuadernillo = small notebook / booklet / little gathering

Then the plural is:

• cuadernillos

So yes, the -illo part is related to the idea of something smaller than a cuaderno, though in technical contexts it may simply be the standard word.

Spanish uses articles more often than English.

In this sentence, el and los make the phrase sound complete and natural:

• El hilo encerado une los cuadernillos.

You might omit an article in some special contexts, like labels, instructions, or very compressed technical writing, but in a normal sentence the articles are expected.

Compare:

• El hilo encerado une los cuadernillos. = normal full sentence
• Hilo encerado une cuadernillos. = sounds more like a heading, note, or telegraphic style

A natural Spain pronunciation would be approximately:

el EE-lo en-seh-RA-do OO-neh los kwa-der-NEE-yos

A few useful details:

• h in hilo is silent
• c before e/i in much of Spain sounds like th in thin, but here in cuadernillos, the c is before u/a, so it sounds like k
• ll in cuadernillos is usually pronounced like y in many modern accents
• stress:
  • HI-lo
  • en-ce-RA-do
  • U-ne
  • cua-der-NI-llos

Yes, unir is a very natural choice here, especially in a technical or descriptive context. It means to join or to connect.

Juntar can also mean to put together or to bring together, but unir often sounds a bit more precise or formal for physical joining.

So:

• El hilo encerado une los cuadernillos. = very natural, especially for describing structure or process
• El hilo encerado junta los cuadernillos. = understandable, but less likely in a technical description

Almost, but not perfectly natural English.

Word by word:

• El = the
• hilo = thread
• encerado = waxed
• une = joins / unites
• los cuadernillos = the booklets / the signatures

So the literal order is:

• The thread waxed joins the booklets

But natural English changes the adjective order:

• The waxed thread joins the booklets.

That adjective placement difference is one of the main things learners notice here.

Yes, depending on context, the Spanish present tense can sometimes correspond to English simple present or present progressive.

So une could be understood as:

• joins
• is joining

However, if Spanish specifically wants to emphasize the ongoing action right now, it would often use:

• está uniendo

In a general description, instruction, or explanation, une is the most natural form.