Em casa, guardei o arroz num frasco e o atum numa lata.

In Portuguese, em casa is a very common idiomatic expression meaning at home or at my/your/our home, depending on context.

A useful contrast is:

• em casa = at home
• na casa = in/at a specific house

So:

• Estou em casa = I’m at home.
• Estou na casa da Ana = I’m at Ana’s house.

In your sentence, Em casa, guardei... simply sets the scene: At home, I put away/stored...

Because Em casa has been moved to the front of the sentence as a setting expression. The comma helps separate that introductory phrase from the main clause.

So the structure is:

• Em casa, = setting/place
• guardei o arroz num frasco e o atum numa lata = main action

You could also say:

• Guardei o arroz num frasco e o atum numa lata em casa.

But starting with Em casa sounds natural if the speaker wants to emphasize where this happened.

Guardei is:

• 1st person singular = I
• pretérito perfeito simples = the simple past / preterite

So guardei means I stored, I put away, or I kept, depending on context.

It comes from the verb guardar.

Examples:

• guardo = I keep / I store
• guardei = I kept / I stored
• guardava = I used to keep / I was keeping

In this sentence, it describes a completed past action.

Because Portuguese often leaves out subject pronouns when the verb ending already makes the subject clear.

Here, guardei can only mean I stored / I put away, so eu is not necessary.

You can say:

• Guardei o arroz... = I stored the rice...
• Eu guardei o arroz... = I stored the rice...

Adding eu usually gives extra emphasis, contrast, or clarity.

For example:

• Eu guardei o arroz, não foi ele. = I stored the rice, not him.

Portuguese uses definite articles much more often than English, especially with nouns referring to food, things, or concepts already understood in context.

So o arroz and o atum are very normal.

This does not always mean the rice and the tuna in a strongly specific English sense. Portuguese simply uses the article more naturally here.

Also:

• arroz is masculine singular → o arroz
• atum is masculine singular → o atum

This is one of those patterns that English speakers usually need to get used to.

They are contractions:

• num = em + um
• numa = em + uma

So:

• num frasco = in a jar
• numa lata = in a can / tin

These contractions are completely standard and very common in both speech and writing.

Similar contractions include:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

Because the nouns have different grammatical genders:

• frasco is masculine → um frasco → num frasco
• lata is feminine → uma lata → numa lata

So the article changes to match the noun.

This is a basic agreement pattern in Portuguese:

• masculine singular: um / o
• feminine singular: uma / a

Because the sentence is talking about a jar and a can, not the jar and the can.

Compare:

• num frasco = in a jar
• no frasco = in the jar

and

• numa lata = in a can
• na lata = in the can

So the choice depends on whether the container is being introduced as indefinite or treated as a specific known one.

They are different kinds of containers.

• frasco usually means a jar, bottle, or small container, often glass or plastic
• lata means a tin or can

So the sentence uses two different nouns because the rice and the tuna are being put into different types of containers.

Depending on context, frasco can sound like a jar with a lid, while lata strongly suggests metal packaging.

Guardar has a wider meaning than just to store.

Common meanings include:

• to keep
• to put away
• to store
• to save
• to guard / watch over in some contexts

In this sentence, guardei most naturally means I put away or I stored.

For example:

• Guardei os livros na estante. = I put the books away on the shelf.
• Guardei dinheiro. = I saved money.
• Guarda isto para mim. = Keep this for me.

Yes, very natural.

The core structure is:

• guardei = verb
• o arroz = first object
• num frasco = where it was put
• e o atum = second object
• numa lata = where that was put

This parallel pattern is neat and very typical:

• o arroz num frasco
• o atum numa lata

Starting with Em casa is also natural because it sets the scene first.

Nothing in the sentence is uniquely European in grammar, but it is perfectly natural in Portugal Portuguese.

A few points that fit well with European Portuguese usage:

• Em casa is extremely common.
• Omitting eu is very normal.
• num / numa are standard and frequent.
• The overall sentence sounds straightforward and natural in Portugal.

So if you are learning Portuguese from Portugal, this is a very good model sentence.