W domu wkładam kopię i oryginał do dużej koperty.

Because w means in/at here and expresses a location, not movement. After w in this meaning, Polish uses the locative case.

• dom = base form
• w domu = in/at home / in the house

If there were movement into the house, you would normally use a different structure, for example do domu = to the house / into the house.

So:

• w domu = location
• do domu = direction toward home

It is the locative singular of dom.

The locative is often used after certain prepositions, especially when talking about a place:

• w domu = in the house / at home
• o domu = about the house

In this sentence, w domu tells us where the action happens.

Because kopię is the accusative singular form of kopia.

Here, kopia is a direct object of wkładam (I put / I am putting in), so it needs the accusative.

• nominative: kopia
• accusative: kopię

This is very common with feminine nouns ending in -a:

• książka → książkę
• herbata → herbatę
• kopia → kopię

Because oryginał is a masculine inanimate noun, and in Polish, masculine inanimate nouns usually have the same form in the nominative and accusative singular.

So:

• nominative: oryginał
• accusative: oryginał

That is why you see:

• kopię changed
• oryginał unchanged

Even though both are direct objects, they belong to different noun classes, so they behave differently.

Because the preposition do requires the genitive case.

The noun is koperta (envelope), and after do it becomes:

• koperta → koperty

The adjective must match the noun in case, gender, and number:

• duża koperta = a big envelope
• do dużej koperty = into a big envelope

So both words change:

• duża → dużej
• koperta → koperty

Because they express two different relationships.

• w domu = at home / in the house → location of the action
• do dużej koperty = into the big envelope → direction/target of the action

So the sentence has both:

1. where the action happens: w domu
2. where something is being put: do dużej koperty

This is very natural in Polish.

Wkładam is the 1st person singular present form of wkładać.

It means something like:

• I put in
• I am putting in
• I insert
• I place into

Because it is an imperfective verb, it can describe:

• a current action: I am putting
• a habitual/repeated action: I put
• a general statement, depending on context

So without extra context, wkładam can mean either I’m putting or I put.

The usual perfective partner is włożyć.

So:

• wkładać = imperfective
• włożyć = perfective

Examples:

• Wkładam kopię do koperty. = I am putting / I put the copy into the envelope.
• Włożę kopię do koperty. = I will put the copy into the envelope.
• Włożyłem kopię do koperty. = I put / have put the copy into the envelope.

This aspect pair is useful because Polish uses aspect very heavily.

Yes, Polish word order is more flexible than English word order.

This sentence could also be said as:

• W domu wkładam kopię i oryginał do dużej koperty.
• Kopię i oryginał wkładam w domu do dużej koperty.
• Do dużej koperty wkładam w domu kopię i oryginał.

These versions all have basically the same core meaning, but the emphasis changes.

Starting with W domu gives a slight focus to the setting: At home, I put the copy and the original into a big envelope.

Because Polish has no articles like English a/an/the.

So kopia can mean:

• a copy
• the copy

and duża koperta can mean:

• a big envelope
• the big envelope

The exact meaning depends on context. In this sentence, English might use the or a, but Polish simply does not need an article.

It can mean either, depending on context.

Very often, w domu is simply the natural Polish way to say at home.

But literally it is also in the house. So the exact English translation depends on the situation:

• Jestem w domu. = I’m at home.
• Siedzę w domu. = I’m sitting at home / in the house.

In your sentence, w domu will usually be understood as at home.

You know it from the noun it describes: koperty comes from koperta, which is a feminine singular noun here in the genitive after do.

The adjective has to agree with the noun, so:

• nominative singular feminine: duża koperta
• genitive singular feminine: do dużej koperty

This kind of agreement is one of the most important patterns in Polish: adjective + noun must match in gender, number, and case.

They are two coordinated direct objects joined by i (and):

• kopię = the copy
• oryginał = the original

Both depend on wkładam.

Polish often allows this kind of mixed-looking pair, where one noun visibly changes and the other does not, because each noun follows its own declension pattern. Grammatically, though, both are in the accusative here.