Telefon jest w worku wodoszczelnym.

Because w means in here, so it shows location. After w in the meaning of location, Polish normally uses the locative case.

• dictionary form: worek
• locative singular: worku

So w worku means in the bag / in a bag.

This is a common pattern with some Polish nouns: the e disappears in some forms.

So:

• worek → nominative singular
• worku → locative singular
• worka → genitive singular

That disappearing e is sometimes called a mobile vowel. You just have to learn that the stem in the other cases is work-.

Because adjectives in Polish must agree with the noun they describe in:

• gender
• number
• case

Here the adjective describes worku, which is:

• masculine
• singular
• locative

So the adjective also has to be masculine singular locative:

• base form: wodoszczelny
• here: wodoszczelnym

It describes the bag, not the phone.

The structure is:

• Telefon = the phone
• jest = is
• w worku wodoszczelnym = in a waterproof bag

A useful clue is agreement. If telefon were being described as waterproof, you would expect wodoszczelny to match telefon directly, for example:

• Telefon jest wodoszczelny = The phone is waterproof.

But here wodoszczelnym matches worku, so it means the bag is waterproof.

Both are understandable, but w wodoszczelnym worku is the more neutral, everyday order.

In Polish, adjectives usually come before the noun:

• w wodoszczelnym worku

Putting the adjective after the noun:

• w worku wodoszczelnym

is possible, but it sounds more marked, formal, or stylistically specific. A learner will usually sound more natural with w wodoszczelnym worku.

Because Polish has no articles.

So Telefon jest w worku wodoszczelnym could mean:

• The phone is in a waterproof bag
• The phone is in the waterproof bag

Context tells you which one is meant.

Telefon is in the nominative singular.

It is the subject of the sentence, the thing that is somewhere:

• Telefon = the phone
• jest = is
• w worku wodoszczelnym = in a waterproof bag

The nominative is also the dictionary form, so telefon stays unchanged here.

Jest is the 3rd person singular of być, meaning is.

So:

• Telefon jest... = The phone is...

In a normal full sentence, you should keep jest. If you say only:

• Telefon w worku wodoszczelnym

that sounds more like a note, label, or headline than a complete neutral sentence.

It is a compound adjective that learners often find easier to remember once they break it apart:

• wodo- = water-related
• szczelny = tight, sealed

So wodoszczelny literally means something like water-tight or water-sealed, which is why it is translated as waterproof.

A helpful breakdown is:

• wo-do-szczel-nym

Approximate English-style guide:

• vo-do-SHCHEL-nim

A few important points:

• Polish w sounds like English v
• szcz is a cluster, roughly like shch
• the stress is normally on the second-to-last syllable, so here on szczel

It does not have to sound perfect right away; the most important part is recognizing the chunk szcz and keeping the stress in the right place.