Como hay holgura, encolamos la unión de caja y espiga.

At the start of a sentence, como can introduce a cause and mean since / because.

So:

• Como hay holgura, ... = Since there is some play/slack, ...

This use is very common when the reason comes before the main clause. If the reason came after, Spanish would more often use porque:

• Encolamos la unión porque hay holgura.

So in this sentence, como is not about manner (how) or comparison (like/as); it is causal.

Because the sentence begins with a causal clause:

• Como hay holgura, = Since there is slack/play,

In Spanish, when this kind of introductory clause comes first, it is normally separated from the main clause with a comma.

So the structure is:

• Como hay holgura, → reason
• encolamos la unión de caja y espiga. → main action

Hay is the impersonal form of haber used to mean there is / there are.

So:

• hay holgura = there is slack/play

Why not está?

• estar usually tells you where something is or what state it is in.
• haber with hay introduces the existence of something.

Compare:

• Hay holgura. = There is some slack/play.
• La holgura está en la unión. = The slack/play is in the joint.

Existe is also possible in some contexts, but it sounds more formal or abstract. Hay is the normal everyday choice.

Because holgura here is being used in a general, uncountable way, like slack, play, or clearance.

• Hay holgura = There is some slack / There is play

This is similar to how Spanish often omits the article with general or non-count nouns after hay:

• Hay agua.
• Hay polvo.
• Hay espacio.

You might say hay una holgura only if you were treating it as a specific, countable instance in a more technical or unusual way. In normal usage, hay holgura sounds more natural.

In woodworking or fitting contexts, holgura usually means play, slack, looseness, or clearance.

It suggests that the fit is not perfectly tight. So here it means the mortise-and-tenon joint has a bit too much room or looseness, and that is why glue is being applied.

It does not mean emotional or figurative ease here, even though holgura can have broader meanings in other contexts.

Grammatically, encolamos can be either:

• present indicative: we glue / we are gluing
• preterite: we glued

That happens because for -ar verbs, the nosotros form looks the same in those two tenses.

For encolar:

• Present: encolamos
• Preterite: encolamos

So how do you know which one it is? You use context.

In this sentence, it is most naturally understood as present, especially if it comes from instructions, a demonstration, or a description of procedure:

• Since there is slack, we glue the mortise-and-tenon joint.

Because encolar is more specific.

• encolar = to glue, especially with adhesive
• pegar = to stick, attach, glue, paste; more general

In woodworking, encolar is a very natural technical verb because it clearly refers to applying glue. Pegar could also be understood, but it is broader and less precise in this context.

So encolamos sounds more like proper workshop vocabulary.

Literally, la unión de caja y espiga is something like:

• the box-and-tenon joint

But that literal translation is misleading, because caja has a technical woodworking meaning here.

In woodworking Spanish:

• caja = mortise
• espiga = tenon

So unión de caja y espiga is the standard type of joint known in English as a mortise-and-tenon joint.

This is a good example of a common word taking on a specialized meaning in a technical field.

Because caja y espiga here functions as the fixed name of a type of joint.

• unión de caja y espiga = mortise-and-tenon joint

When Spanish names a class or type of object, it often uses de + noun(s) without articles:

• mesa de madera
• gafas de sol
• máquina de coser

Likewise:

• unión de caja y espiga

If you said de la caja y la espiga, it would sound more like you were talking about a particular box and a particular tenon, not the name of the joint type.

Very often in Spanish instructions, demonstrations, tutorials, or workshop talk, nosotros forms are used as an inclusive or instructional we:

• Ahora cortamos la pieza.
• Después lijamos el borde.
• Luego encolamos la unión.

This does not always mean a specific group of people is literally doing it together. It can mean something like:

• we do this now
• the next step is to...
• you/one would do this

So here encolamos may simply be the instructional style commonly used in explanations.

Yes, but the structure would usually change.

Natural options would be:

• Como hay holgura, encolamos la unión de caja y espiga.
• Encolamos la unión de caja y espiga porque hay holgura.

Both are correct, but they are arranged differently:

• como often introduces the reason first
• porque often introduces the reason after the main clause

Starting with como can sound slightly more like given that / since.

Other words can exist, but unión is a perfectly normal and clear choice.

Depending on region and technical style, you might also see words such as:

• ensamble
• ensamblaje
• junta in some contexts

But la unión de caja y espiga is easy to understand and idiomatic in technical or workshop language. It focuses on the fact that this is a joint/connection between two pieces.

Because the sentence uses the full noun phrase as the direct object:

• encolamos la unión de caja y espiga

Spanish can replace that with a pronoun if the object is already known:

• Como hay holgura, la encolamos.

But when introducing the action clearly, especially in instructional language, Spanish often keeps the full noun phrase instead of switching to a pronoun too soon. That makes the sentence more explicit.