Guarda el frasco de mermelada en la nevera, por favor.

Guarda is the affirmative tú command of guardar: guarda = put away / keep / store.

So the sentence is speaking to one person informally:

• (tú) guarda = put it away / keep it

Compare:

• guardas = you keep / you store (present tense statement, not a command)
• guarde = formal singular command (usted) or present subjunctive

So:

• Guarda el frasco... = informal, to one person
• Guarde el frasco... = formal, to one person

For most regular verbs, the affirmative tú command is the same as the él/ella/usted form of the present tense.

With guardar:

• yo guardo
• tú guardas
• él/ella guarda

So the tú command is:

• guarda

This is very common:

• hablar → habla
• comer → come
• vivir → vive
• guardar → guarda

Spanish often uses the article el where English uses the:

• el frasco = the jar

That part is straightforward. What may feel different is that Spanish often keeps nouns more explicit and concrete in everyday instructions.

Here:

• el frasco de mermelada = the jar of jam

It identifies exactly which jar you mean. In context, a speaker could shorten it if it were obvious:

• Guarda el frasco en la nevera.
• Guárdalo en la nevera.

But the full phrase is perfectly natural.

De is the normal way to describe what a container is for, contains, or is associated with.

So:

• frasco de mermelada = jar of jam
• botella de agua = bottle of water
• taza de café = cup of coffee

Using con would focus more on with in a more literal sense:

• un frasco con mermelada = a jar with jam in it

That is possible, but less idiomatic here. In everyday Spanish, frasco de mermelada is the natural choice.

In Spanish, when one noun describes another noun like jar of jam, the connection is usually made with de.

English can stack nouns directly:

• jam jar

Spanish usually does not do that in the same way. Instead it says:

• frasco de mermelada

This structure is extremely common:

• vaso de leche = glass of milk
• lata de atún = can of tuna
• tarro de miel = jar of honey

Nevera is very common in Spain for fridge / refrigerator.

In other Spanish-speaking countries, people may also say:

• refrigerador
• refrig­erador
• heladera

For Spain Spanish, nevera is one of the most natural choices.

So:

• en la nevera = in the fridge

If you are focusing on Spain, nevera is an excellent word to learn.

Because the idea here is location, not movement toward a place in the same way Spanish often marks with a.

• en la nevera = in the fridge

With guardar, Spanish normally uses en to show where something is placed/stored:

• Guarda la leche en la nevera.
• Guardó los documentos en un cajón.

English says put it in the fridge, and Spanish matches that with en.

Yes. Meter is very common in everyday Spanish for put / stick / place inside.

So:

• Guarda el frasco de mermelada en la nevera = put/store the jam jar in the fridge
• Mete el frasco de mermelada en la nevera = put the jam jar in the fridge

The difference is small:

• guardar often suggests put away / store properly
• meter focuses more on putting something inside

Both can work, but guardar sounds a bit more like put it away where it belongs.

Yes. Depending on the object and region, you may hear:

• frasco
• tarro
• bote

All can refer to a kind of jar/container, though nuance depends on context.

In Spain:

• bote de mermelada and tarro de mermelada are also very natural
• frasco can sound a bit more like a glass container or a jar/bottle, depending on context

So this sentence is correct, but other versions may also sound natural:

• Guarda el bote de mermelada en la nevera.
• Guarda el tarro de mermelada en la nevera.

Por favor is flexible in Spanish. It can go at the beginning, middle, or end.

All of these are natural:

• Por favor, guarda el frasco de mermelada en la nevera.
• Guarda, por favor, el frasco de mermelada en la nevera.
• Guarda el frasco de mermelada en la nevera, por favor.

Putting it at the end is very common and sounds polite and natural.

It is informal, because of the command guarda, which is for tú.

If you wanted the formal version for one person, you would say:

• Guarde el frasco de mermelada en la nevera, por favor.

If you were speaking to more than one person in Spain:

• informal plural: Guardad el frasco de mermelada en la nevera, por favor.
• formal plural: Guarden el frasco de mermelada en la nevera, por favor.

Yes. Since el frasco is masculine singular, you can replace it with lo.

So:

• Guarda el frasco de mermelada en la nevera. becomes:
• Guárdalo en la nevera.

Notice the accent:

• guarda + lo → guárdalo

Spanish adds the written accent to keep the normal stress.

This is a very common structure with affirmative commands:

• coge + lo → cógelo
• abre + la → ábrela
• pon + los → ponlos

Spanish usually omits subject pronouns when they are not needed, because the verb form already shows who the subject is.

Here, guarda already tells you it is an instruction to tú.

So:

• Guarda el frasco... and
• Tú guarda el frasco... both point to you, but the first is the normal choice.

Including tú would usually add emphasis, contrast, or emotion:

• Tú guarda el frasco; yo me encargo del resto.

A more internationally recognizable version might be:

• Guarda el frasco de mermelada en el refrigerador, por favor.

But for Spain, nevera is usually more natural in everyday speech.

So if your goal is Spanish from Spain, the original sentence is a very good model:

• Guarda el frasco de mermelada en la nevera, por favor.