In meine Brotdose packe ich heute Brot, einen Apfel und etwas Käse.

Because German main clauses follow the verb-second rule: the finite verb must come in the second position, but the first position can be filled by many different kinds of information.

So in this sentence:

In meine Brotdose | packe | ich | heute | ...

the speaker chooses to put In meine Brotdose first to emphasize where the food is going.

A more neutral version would be:

Ich packe heute Brot, einen Apfel und etwas Käse in meine Brotdose.

Both are correct. The version with In meine Brotdose first sounds a bit more like: Into my lunchbox, I’m packing...

Because once In meine Brotdose is placed in the first position, the conjugated verb must still stay in the second position.

So German word order becomes:

• 1st position: In meine Brotdose
• 2nd position: packe
• then the subject: ich

This is very common in German:

• Heute gehe ich nach Hause.
• In Berlin wohnt er.
• Morgen kaufen wir Brot.

So packe ich is not strange word order here; it is exactly what German grammar requires.

Because in can take either the accusative or the dative, depending on the meaning.

• accusative = movement/change of location, answering where to?
• dative = location, answering where?

Here, the food is being packed into the lunchbox, so there is movement toward a destination. That is why German uses the accusative:

• in meine Brotdose = into my lunchbox

If you were describing where something already is, you would use the dative:

• Das Brot ist in meiner Brotdose. = The bread is in my lunchbox.

So:

• in meine Brotdose = motion into it
• in meiner Brotdose = located inside it

Brotdose is a feminine noun: die Brotdose.

That is why the possessive changes accordingly:

• nominative: meine Brotdose
• accusative: meine Brotdose
• dative: meiner Brotdose

In this sentence, the noun is feminine singular and used after in with the accusative, so meine is the correct form.

Also, Brotdose is a compound noun:

• Brot = bread
• Dose = container/tin/box

In German compounds, the last part determines the gender, so because Dose is feminine, Brotdose is feminine too.

Because Apfel is a countable noun, while Brot and Käse are being used here as uncountable food substances.

• einen Apfel = one apple
• Brot = bread, some bread
• Käse = cheese, some cheese

In English, we do the same thing:

• an apple
• bread
• cheese

The form einen appears because:

• Apfel is masculine: der Apfel
• it is in the accusative singular
• so ein becomes einen

Compare:

• nominative: ein Apfel
• accusative: einen Apfel

Etwas here means some or a bit of.

So etwas Käse means:

• some cheese
• a little cheese

It is very common with uncountable nouns:

• etwas Wasser = some water
• etwas Zeit = some time
• etwas Geld = some money

In this sentence, it helps show that the speaker is not talking about a whole specific cheese, but simply some amount of cheese.

Yes. They are all things being packed, so they function as direct objects and belong to the accusative area of the sentence.

However, not every noun shows a visible change in form.

• Brot: no visible article here, so you do not see a change
• einen Apfel: visible accusative marking on einen
• etwas Käse: etwas does not change like ein/einen, so again there is no obvious accusative ending

So even though only einen Apfel clearly shows accusative form, all three items are objects of packe.

Yes, heute can move around, because adverb placement in German is somewhat flexible.

In this sentence:

In meine Brotdose packe ich heute Brot, einen Apfel und etwas Käse.

heute fits naturally after the subject.

But other versions are also possible:

• Heute packe ich Brot, einen Apfel und etwas Käse in meine Brotdose.
• Ich packe heute Brot, einen Apfel und etwas Käse in meine Brotdose.

The version you have puts the focus first on where the food is going, and then adds heute as the time reference.

So the placement is natural, but not the only possible one.

Yes. Brotdose is a very normal everyday German word, especially in Germany.

Literally it means something like bread box/container, but in real use it often means:

• lunchbox
• food container
• a box for taking food with you

Depending on context, people might also say:

• Lunchbox
• Vesperdose (regional)
• Brotbox

But Brotdose is absolutely standard and natural.

Yes, definitely. That is a completely correct and very natural sentence.

The difference is mainly focus:

• Ich packe heute ... in meine Brotdose. = more neutral
• In meine Brotdose packe ich heute ... = stronger emphasis on the destination

German often moves sentence elements to the first position to highlight them. So both versions are grammatical; they just organize the information differently.