Mój telefon jest w torbie, ale nie mogę bez niego pracować.

Because mój has to agree with telefon in gender, number, and case.

• telefon is masculine singular
• it is the subject here, so it is in the nominative
• the matching form of my is mój

Compare:

• mój telefon — my phone
• moja torba — my bag
• moje okno — my window

So mój telefon is the correct match.

Because Polish does not have articles like a/an and the.

So Polish usually just says:

• telefon — phone / the phone / a phone

The exact meaning depends on context. In this sentence, mój telefon already clearly means my phone, so no article is needed.

Yes, in a normal sentence like this, jest is the standard form.

• Mój telefon jest w torbie = My phone is in the bag.

Polish normally does use the present tense of być (to be) in this kind of sentence.
If you say just Mój telefon w torbie, it sounds incomplete or very elliptical, like a note, headline, or casual shorthand.

So for standard Polish, jest should stay.

Because after w meaning in for a location, Polish uses the locative case.

The basic form is:

• torba — bag

But after w when you mean in the bag, it changes to:

• w torbie

So:

• w domu — in the house
• w pracy — at work
• w torbie — in the bag

This is one of the most important things to learn in Polish: prepositions often change the form of the noun.

It is the locative singular.

Here is the pattern:

• torba — nominative
• w torbie — locative after w for location

So in this sentence:

• telefon is in the nominative because it is the subject
• torbie is in the locative because it follows w

Mogę is the 1st person singular form of móc, which means can / to be able to.

So:

• mogę — I can
• możesz — you can
• może — he/she/it can
• możemy — we can

With negation:

• nie mogę — I cannot / I can’t

So nie mogę matches the implied subject I.

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

• mogę already means I can
• so ja is not necessary

You can add ja for emphasis:

• Ja nie mogę bez niego pracować — I can’t work without it/him

But in a neutral sentence, Polish usually leaves ja out.

Because after móc (can / be able to), Polish uses the infinitive of the second verb.

So:

• mogę pracować — I can work
• nie mogę pracować — I can’t work

This is similar to English:

• I can work
• not I can working

So after mogę, you use pracować, not a conjugated form like pracuję.

Because bez (without) requires the genitive case.

The pronoun referring to telefon therefore has to be in a genitive form:

• on — he / it
• jego / go — him / it
• after a preposition, Polish normally uses the form with n-
• so: bez niego

That is why you get:

• bez niego — without him / without it

Not:

• bez nim — wrong here, because nim is not the genitive form
• bez go — also wrong after this preposition

This is a very common Polish pattern: after many prepositions, pronouns take forms like niego, niej, nich.

Grammatically, niego can refer to a masculine noun, so depending on context it can mean him or it.

Here it refers back to:

• telefon — a masculine noun

So in this sentence, bez niego means:

• without it

Even though the form is the same one you might also translate as without him in another sentence.

Yes. Polish word order is fairly flexible, though some versions sound more natural or put emphasis in different places.

The original sentence:

• Mój telefon jest w torbie, ale nie mogę bez niego pracować.

You could also say:

• Mój telefon jest w torbie, ale nie mogę pracować bez niego.

Both are correct. The difference is mostly emphasis:

• nie mogę bez niego pracować puts without it a bit earlier and may stress dependence on the phone
• nie mogę pracować bez niego sounds slightly more neutral in some contexts

Polish often moves parts around for focus, not because the grammar changes.

A few helpful points:

• ó in mój is pronounced like u
• j in Polish sounds like English y in yes
• so mój sounds roughly like mooy

For mogę:

• g is always a hard g
• final ę is nasal, though in everyday speech it is often pronounced less strongly at the end of a word
• so mogę sounds roughly like MO-gę, with a nasal ending

For niego:

• nie sounds roughly like nye
• so niego is roughly NYE-go

These are only approximations, but they can help at the start.