Guarda el rotulador en el cajón para no perderlo.

Guarda is the affirmative tú command of guardar.

It is used when telling one person informally to do something: Put away / keep the marker.

Related forms:

• guarde = formal singular (usted)
• guardad = informal plural in Spain (vosotros)
• guarden = formal plural (ustedes)

Because Spanish usually omits subject pronouns when they are already clear from the verb form.

So:

• Guarda el rotulador... is the normal way
• Tú guarda el rotulador... is possible, but sounds emphatic or contrastive

English needs you much more often than Spanish does.

Guardar often means to put away, store, keep somewhere safe.
Poner just means to put/place.

In this sentence, guardar is very natural because the second part is para no perderlo. The idea is not just placing the marker somewhere, but putting it away so it won’t get lost.

Spanish often uses the definite article when the object is specific, known from context, or treated as the obvious one.

So el rotulador and el cajón can sound natural even where English might sometimes say a marker or a drawer.

You could change the article, but the meaning shifts:

• el rotulador = the marker, the one we mean
• un rotulador = a marker, any marker
• el cajón = the drawer, a particular drawer
• un cajón = a drawer, any drawer

Yes, rotulador is very common in Spain for a marker / felt-tip pen.

In many parts of Latin America, people are more likely to say things like:

• marcador
• plumón
• other regional words depending on the country

So if you are learning Spanish from Spain, rotulador is exactly the kind of word you should expect.

Because with guardar, Spanish normally uses en to indicate the place where something is kept:

• guardar algo en un cajón
• guardar algo en una caja
• guardar algo en el bolso

So en el cajón simply means in the drawer.

Para + infinitive expresses purpose:

• para guardar = in order to put away
• para encontrar = in order to find

When you add no, it becomes negative:

• para no perderlo = in order not to lose it / so as not to lose it

So this part gives the reason or goal behind the command.

Because para expresses purpose, while porque expresses cause/reason.

Here the meaning is:

• Put the marker in the drawer so that you don’t lose it

That is a purpose, so Spanish uses para.

If you used porque, the sentence structure and meaning would be different.

Lo refers back to el rotulador.

It is the masculine singular direct object pronoun, so it means it.

Why masculine? Because rotulador is a masculine noun:

• el rotulador → lo
• if it were la libreta, the pronoun would be la

Because perder also needs its own object.

Spanish can either:

• repeat the noun: para no perder el rotulador
• or replace it with a pronoun: para no perderlo

The second option is usually more natural and less repetitive.

So lo is there because it is the object of perder, not because Spanish is randomly repeating information.

Because perder is an infinitive, and in Spanish object pronouns can attach to infinitives.

So:

• perder + lo → perderlo

That is completely normal.

Compare:

• Quiero verlo
• Voy a comprarlo
• Para no perderlo

If the verb were conjugated, the pronoun would usually go before it:

• para que no lo pierdas

Yes. That is also correct, and very natural if the marker is already understood from context.

• Guarda el rotulador en el cajón = says the noun explicitly
• Guárdalo en el cajón = uses a pronoun instead

Notice the accent mark in Guárdalo. It is added to keep the normal stress after attaching lo to the command.

Because Spanish stress rules would otherwise place the stress differently.

A word ending in -n, -s, or a vowel is normally stressed on the second-to-last syllable.
Without the accent, cajon would be pronounced with the stress earlier.

But the real stress is on the last syllable:

• ca-JÓN

So the written accent in cajón shows that irregular stress.