Guarda el termo en el estante de arriba, por favor.

Because guarda is the affirmative tú command of guardar.

In this sentence, the speaker is telling someone informally, Put/keep/store the thermos...

A quick comparison:

• guardar = to put away, to store, to keep
• guardas = you keep / you store
• guarda = keep / put away! (command to tú)

So:

• Guarda el termo... = Put the thermos away...

It is understood, not stated.

Spanish often leaves out subject pronouns when they are clear from the verb form. Here, guarda already shows that the command is directed to tú.

So the full idea is:

• (Tú) guarda el termo en el estante de arriba, por favor.

But Spanish normally omits tú unless you want emphasis, contrast, or clarity.

Here, guardar means something like:

• to put away
• to store
• to put
• sometimes to keep

In this sentence, the most natural English idea is probably put the thermos on the top shelf or put the thermos away on the upper shelf.

It does not necessarily mean guard in the English sense, even though the words look related.

El termo usually means a thermos, vacuum flask, or insulated container for keeping drinks hot or cold.

In Spain, termo is understandable and common. Depending on context, people might also say:

• botella térmica = insulated bottle
• termo = thermos / flask

So in this sentence, el termo is simply the thermos.

El means the, while un means a/an.

The sentence uses el termo because the speaker is referring to a specific thermos, probably one both people know about.

• Guarda el termo... = Put away the thermos...
• Guarda un termo... = Put away a thermos...

Spanish uses articles very often, and here the definite article sounds natural because it is a particular object.

Literally, it is:

• en = in / on
• el estante = the shelf
• de arriba = of above / upper / on top

So the natural meaning is:

• on the top shelf
• on the upper shelf
• on the shelf above

Spanish often uses de arriba to mean upper or the one above.

Because de arriba works like the upper one or the one above.

Compare these:

• el estante de arriba = the top shelf / the shelf above
• arriba by itself = above / upstairs / up

If you said en el estante arriba, that would sound wrong or incomplete in standard Spanish.

The structure noun + de arriba is very common:

• la caja de arriba = the box on top / the upper box
• el piso de arriba = the upstairs flat/apartment / the floor above

Because Spanish often uses en where English uses either in or on.

So:

• en el estante = on the shelf

This is normal. Spanish en covers a wider range than English in.

Other examples:

• en la mesa = on the table
• en la pared = on the wall
• en el cajón = in the drawer

So you should not translate en too literally every time.

It is perfectly correct and natural, but there are other words too.

In Spain, you may also hear:

• estante = shelf
• balda = shelf board, shelf
• repisa = ledge / shelf, sometimes more like a wall shelf

So el estante de arriba is fine, but in some contexts a Spaniard might also say:

• la balda de arriba

That can sound especially natural when talking about one shelf level in a cupboard or shelving unit.

Because por favor is flexible in Spanish. It can go at the beginning, middle, or end of a polite request.

All of these are possible:

• Por favor, guarda el termo en el estante de arriba.
• Guarda, por favor, el termo en el estante de arriba.
• Guarda el termo en el estante de arriba, por favor.

Putting it at the end is very common and natural.

You would use the usted command instead of the tú command.

• Guarde el termo en el estante de arriba, por favor.

Here:

• guarda = informal singular command (tú)
• guarde = formal singular command (usted)

Other useful versions:

• Guardad el termo... = informal plural in Spain (vosotros)
• Guarden el termo... = formal plural / Latin American plural (ustedes)

Yes. Since el termo is the direct object, you can replace it with lo.

That gives:

• Guárdalo en el estante de arriba, por favor.

This means:

• Put it on the top shelf, please.

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

So both are correct:

• Guarda el termo en el estante de arriba, por favor.
• Guárdalo en el estante de arriba, por favor.

A simple pronunciation guide is:

• GWAR-da el TER-mo en el es-TAN-te de a-RRI-ba, por fa-BOR

A few notes:

• gua in guarda sounds roughly like gwa
• r in termo is a tapped r
• rr in arriba is a stronger rolled sound, but here there is only one r in arriba? Actually, arriba is spelled with rr, so it has the trilled sound: a-RRI-ba
• v and b are pronounced very similarly in Spanish
• In most of Spain, z and soft c have the th sound of think, but this sentence does not contain those letters

A broad IPA version would be approximately:

• [ˈɡwarða el ˈteɾmo en el esˈtante ðe aˈriβa poɾ faˈβoɾ]

Yes, it is natural and correct.

A Spaniard would understand it immediately. Depending on the exact object and the type of shelf, someone might also say things like:

• Pon el termo en el estante de arriba, por favor.
• Guarda el termo en la balda de arriba, por favor.

The original sentence is still completely normal and idiomatic.