Guarda el pasaporte a mano, por si te lo piden en la aduana.

Guarda is the affirmative tú command of guardar.

So the verb forms are:

• guardar = to keep, store, put away

• guarda = keep / put away — when speaking to one person informally

  In Spain, this is the normal way to tell one person casually what to do.

Compare:

• Guarda el pasaporte = Keep the passport handy
• No guardes el pasaporte ahí = Don’t put the passport there
• Guarde el pasaporte = formal singular command
• Guardad el pasaporte = plural command used with vosotros in Spain

Yes. Guardar often means to store, put away, save, or keep, and the exact meaning depends on context.

In this sentence, because of a mano, it means something like:

• keep it accessible
• keep it handy
• have it ready

So this is not the idea of putting the passport deep in a bag. It is more like: make sure it is somewhere easy to reach.

Spanish often uses the definite article (el, la, los, las) where English would use a possessive like your or my, especially when it is already obvious whose thing it is.

Here, the context makes it clear that it means your passport.

So:

• Guarda el pasaporte a mano sounds natural
• Guarda tu pasaporte a mano is also possible, but it is a bit more explicit

Spanish does this a lot with personal belongings and body parts:

• Lávate las manos = Wash your hands
• Ponte el abrigo = Put on your coat

A mano is an idiomatic expression meaning:

• at hand
• handy
• within easy reach

So guardar algo a mano means to keep something where you can get it quickly.

Examples:

• Ten el móvil a mano = Keep your phone handy
• Deja las llaves a mano = Leave the keys where they’re easy to reach

It does not literally mean that it must be in your hand.

Because the meaning is different.

• a mano = handy, accessible, easy to reach
• en la mano = in your hand physically

So:

• Guarda el pasaporte a mano = Keep the passport handy
• Lleva el pasaporte en la mano = Carry the passport in your hand

At customs, you do not need to be holding it every second. You just need to be able to get it quickly.

Por si means in case.

It introduces a precaution: you do something now because something might happen later.

So here, the idea is:

• keep the passport handy in case they ask for it

This is different from plain si, which means if and introduces a condition.

Compare:

• Llévalo por si lo necesitan = Take it in case they need it
• Si lo necesitan, llévalo = If they need it, take it

With por si, the speaker is talking about preparing for a possibility.

Because por si normally takes the indicative, not the subjunctive.

So Spanish says:

• por si viene = in case he/she comes
• por si llaman = in case they call
• por si te lo piden = in case they ask you for it

This can feel surprising to English speakers who expect a more hypothetical form, but after por si, standard Spanish uses the indicative because the speaker is referring to a real possible situation, not expressing doubt in the same way the subjunctive would.

Te lo contains two object pronouns:

• te = to you or from you
• lo = it, referring to el pasaporte

With pedir, Spanish often expresses both the thing requested and the person it is requested from.

So:

• pedir algo = to ask for something
• pedir algo a alguien = to ask someone for something
• te lo piden = they ask you for it

The order is important:

• indirect object pronoun first: te
• direct object pronoun second: lo

So te lo is correct, while lo te is not.

Because el pasaporte is the direct object of the verb pedir, and lo is the standard direct object pronoun for a masculine singular thing.

So:

• el pasaporte → lo

Meanwhile, te refers to the person involved.

In other words:

• te = from you / to you
• lo = the passport

That is why te lo piden means they ask you for it.

The subject is an implied they.

Spanish often leaves subject pronouns out when the verb ending already shows the person and number. So piden by itself already means they ask.

In this context, the hidden they would usually be:

• customs officers
• border staff
• officials

So the sentence does not need to say ellos. It is understood from the situation.

La aduana means customs, meaning the customs area or customs control at an airport, port, or border.

So:

• en la aduana = at customs

Even though English uses customs, which looks plural, Spanish uses the singular noun la aduana.

You may also hear related expressions like:

• pasar por la aduana = to go through customs
• control de aduanas = customs control

In this sentence, it simply tells you where they might ask for the passport.