Śledzę przesyłkę, która zaginęła, i chcę złożyć reklamację.

Because przesyłkę is in the accusative case.

The verb śledzić means to track / to follow, and it takes a direct object. In Polish, a feminine singular noun like przesyłka changes to przesyłkę in the accusative:

• przesyłka = nominative, dictionary form
• przesyłkę = accusative, used here after śledzę

So:

• Śledzę przesyłkę = I am tracking the shipment

Która means which / that here, and it refers back to przesyłka.

It agrees with przesyłka in:

• gender: feminine
• number: singular

That is why it is która, not:

• który (masculine)
• które (neuter)

A very important point: the case of która is decided by its role inside its own clause, not by the case of przesyłkę in the main clause.

In która zaginęła, która is the subject of zaginęła, so it is in the nominative.

Because Polish past tense agrees with the subject in gender and number.

The subject here is która, referring to przesyłka, which is feminine singular. So the verb takes the feminine singular past ending:

• zaginął = masculine singular
• zaginęła = feminine singular
• zaginęło = neuter singular
• zaginęli / zaginęły = plural

So która zaginęła literally means which went missing.

It means went missing, got lost, or disappeared.

For a parcel or shipment, zaginąć is a very natural verb. It suggests that the item has gone missing in transit or can no longer be located.

It is more natural here than something like zgubiła się. For shipments, zaginęła sounds more standard and idiomatic.

Złożyć reklamację is a fixed, common expression meaning:

• to file a complaint
• to submit a claim
• to lodge a complaint

Literally, złożyć often means to put together / to submit, but in this context it means to formally submit something.

So:

• chcę złożyć reklamację = I want to file a complaint / claim

This is the normal formal phrase used with deliveries, products, services, billing issues, and so on.

No. This is a false friend.

In Polish, reklamacja usually means:

• complaint
• claim
• a formal complaint about a product, service, or delivery

It does not usually mean English reclamation.

Also, it has nothing to do with advertising, even though it looks a bit like reklama.

Because Polish often drops subject pronouns when they are clear from the verb form.

Here, both verbs already show first person singular:

• śledzę = I track / I am tracking
• chcę = I want

So ja is unnecessary unless you want extra emphasis, contrast, or emotion:

• Ja śledzę przesyłkę... = I’m the one tracking the shipment...

Without ja, the sentence sounds more neutral and natural.

Because in Polish, relative clauses introduced by words like który, która, które are normally separated by commas.

So:

• Śledzę przesyłkę, która zaginęła, i chcę złożyć reklamację.

The clause która zaginęła sits in the middle of the sentence, so it gets:

• one comma before it
• one comma after it

This is more regular in Polish than in English. In English, punctuation with that/which can vary; in Polish, the commas are standard here.

Because the speaker is describing what they are doing now:

• Śledzę przesyłkę = I am tracking the shipment
• która zaginęła = which has gone missing / which went missing

So the sentence combines:

• a current action: I’m tracking
• a past event: it went missing
• a current intention: I want to file a complaint

That combination is perfectly natural.

This sentence is a nice example of Polish aspect.

• śledzę comes from śledzić, which is imperfective
  It presents the action as ongoing or in progress: I am tracking

• zaginęła comes from zaginąć, which is perfective
  It presents the disappearance as a completed event: it went missing

• złożyć is also perfective
  It presents filing the complaint as a single complete act: to file / submit

Why chcę złożyć, not chcę składać?

Because złożyć reklamację focuses on completing the action once.
Składać reklamację would sound more process-based, repeated, or less natural here.

Yes, Polish word order is fairly flexible, but the original sentence is very natural and neutral.

The original:

• Śledzę przesyłkę, która zaginęła, i chcę złożyć reklamację.

puts the actions in a straightforward order:

1. I’m tracking the shipment
2. the shipment went missing
3. I want to file a complaint

You could rearrange parts for emphasis, but the default version is a good one for learners to copy.

A few sounds here are especially important:

• ś in Śledzę is a soft sound, somewhat like a softer English sh
• ł in przesyłkę is pronounced like English w
• ę is a nasal vowel, but at the end of a word or before a pause it is often pronounced less strongly nasal than learners expect

Rough guides:

• Śledzę ≈ shled-zeh with a soft sh
• przesyłkę ≈ pshe-SIW-keh
• chcę ≈ something like h-tseh with a nasal ending

These are only approximations. The most important things to remember are:

• ś is soft
• ł sounds like w
• final ę often sounds close to e, especially in everyday speech

They overlap, but they are not always exactly the same.

• przesyłka = shipment / delivery / postal item
• paczka = package / parcel

Przesyłka is a bit broader and more official-sounding. It works very well in contexts like tracking, delivery services, logistics, and complaints.

So in this sentence, przesyłka sounds very natural, especially if you are talking about courier tracking and filing a complaint.

You might hear similar phrasing in some contexts, but chcę złożyć reklamację is the more standard and natural formal expression.

Compare:

• złożyć reklamację = to file a complaint / submit a claim
• reklamować coś = to make a complaint about something / claim under warranty

For a lost shipment, złożyć reklamację is the safest, most idiomatic choice.