Mi padre guarda otro tapón en el cajón por si este se pierde.

In Spanish, otro normally does not take un before it.

So you say:

• otro tapón = another stopper/cap
• otra vez = again / another time
• otro día = another day

Using un otro tapón would sound unnatural in standard Spanish.

Tapón can mean several things depending on context, such as:

• stopper
• cap
• plug
• cork

In this sentence, it means some kind of object used to close something. The exact English word depends on what is being closed: a bottle, a container, a sink, etc.

Guarda is the present simple of guardar.

Spanish often uses the simple present where English might also use:

• keeps
• stores
• puts away
• sometimes even is keeping

Here, Mi padre guarda otro tapón... suggests a usual or practical habit: he keeps another one in the drawer.

Using está guardando would sound more like something happening right now, at this moment: My father is putting another stopper in the drawer right now.

Here guardar means something like:

• to keep
• to store
• to put away

So Mi padre guarda otro tapón en el cajón means your father keeps or stores another stopper/cap in the drawer, not necessarily that he is actively guarding it.

Even though guardar can sometimes be translated as to guard, in everyday Spanish it very often means to keep or to put away.

En means in or inside, and el cajón means the drawer.

So:

• en el cajón = in the drawer

The definite article el is used because the speaker is referring to a specific drawer, probably one already understood from the context.

If you said en un cajón, that would mean in a drawer, with less specificity.

Por si means in case.

It introduces a precaution: someone does something now because something else might happen later.

So:

• guarda otro tapón en el cajón por si este se pierde
• he keeps another stopper in the drawer in case this one gets lost

A useful thing to remember is:

• por si = in case

Because por si expresses a precaution or preparation.

Compare:

• si = if
• por si = in case

So this sentence is not saying if this one gets lost, then... It is saying he keeps another one now in case this one gets lost later.

That idea of taking precautions is exactly what por si is for.

Este means this one here, and it refers back to the original tapón.

So the idea is:

• He keeps another stopper in the drawer
• in case this one gets lost

It helps distinguish between:

• otro tapón = the spare one
• este = the current one

Without este, the meaning might still be understood from context, but este makes the contrast clearer.

Modern standard Spanish normally writes demonstratives like este, ese, aquel without an accent mark, even when they stand alone.

So:

• este = correct modern spelling

Older texts, older teaching materials, or some writers may use éste, but today the usual recommendation is este.

Se pierde here means gets lost or is lost.

The verb is perderse, which often means:

• to get lost
• to become lost
• to go missing

So:

• por si este se pierde = in case this one gets lost

This is more natural than translating it word-for-word as loses itself.

Because Spanish often uses the pronominal form perderse to mean to get lost.

Compare:

• perder algo = to lose something
  • Perdí las llaves. = I lost the keys.
• perderse = to get lost / be lost
  • Las llaves se perdieron. = The keys got lost.

In your sentence, the stopper is the thing that may go missing, so Spanish uses se pierde.

After por si, standard Spanish normally uses the indicative, not the subjunctive.

So you say:

• por si este se pierde
• por si llueve
• por si no viene

This is different from some other expressions of possibility or purpose that do use the subjunctive.

A good rule for learners is:

• por si + indicative

You could change the order in some contexts, but the original is the most natural.

• por si este se pierde sounds normal and clear
• por si se pierde este is possible, but less neutral
• por si pierde este is not natural for this meaning

The original order works well because este is the subject of se pierde.

Yes, sometimes you could say:

• Mi padre guarda otro tapón en el cajón por si se pierde.

If the context is clear, Spanish can omit the explicit subject. But in this sentence, este is useful because it clearly points to this one, meaning the stopper currently in use, not the spare one.

So omitting it is possible, but keeping it makes the meaning more precise.

Yes, it is natural in Spain.

Nothing in the sentence is especially strange or strongly regional. Words like padre, guardar, cajón, and por si are completely standard.

The exact meaning of tapón may depend on the object involved, but the sentence itself sounds like normal standard Spanish, including in Spain.