Mój plecak jest cięższy niż twoja torba.

Because adjectives and possessive words in Polish must agree with the noun’s gender, number, and case.

• plecak (backpack) is masculine singular, so you use mój = my
• torba (bag) is feminine singular, so you use twoja = your

That is why the sentence has:

• mój plecak
• twoja torba

Other forms of my include, for example:

• moja for feminine singular
• moje for neuter singular

Because cięższy is the comparative form of ciężki (heavy).

• ciężki = heavy
• cięższy = heavier

The sentence is making a comparison, so Polish uses the comparative form, just like English uses heavier instead of heavy.

Also, cięższy agrees with plecak, which is masculine singular.

It is formed by making a comparative, but the stem changes a little.

• ciężki → cięższy

This is not just a simple add-on like English -er. In Polish, comparatives often involve:

• dropping or changing part of the adjective ending
• adding -szy or -ejszy

So this is something learners usually memorize as a comparative form.

A few examples:

• mały → mniejszy = smaller
• szybki → szybszy = faster
• ciężki → cięższy = heavier

Niż means than in comparisons.

So:

• cięższy niż = heavier than

In this sentence:

• Mój plecak jest cięższy niż twoja torba.
• My backpack is heavier than your bag.

This is the standard word used for direct comparisons.

Yes, very often.

You can say:

• Mój plecak jest cięższy niż twoja torba.
• Mój plecak jest cięższy od twojej torby.

Both mean the same thing: My backpack is heavier than your bag.

But there is an important grammar difference:

• after niż, the noun phrase often stays in the same basic form: twoja torba
• after od, the following noun phrase must go into the genitive case: twojej torby

So:

• niż twoja torba
• od twojej torby

Jest means is.

In careful, neutral Polish, including jest is completely normal:

• Mój plecak jest cięższy niż twoja torba.

In everyday speech, Polish sometimes omits jest when the meaning is obvious:

• Mój plecak cięższy niż twoja torba.

But that sounds more informal, shortened, or conversational. For learners, it is safest to use jest.

Because it describes plecak, and plecak is a masculine noun.

In Polish, adjectives must agree with the noun they describe.

So if the noun changed, the adjective form would change too:

• plecak (masculine) → cięższy
• torba (feminine) → cięższa
• krzesło (neuter) → cięższe

For example:

• Ta torba jest cięższa. = This bag is heavier.
• To krzesło jest cięższe. = This chair is heavier.

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

The neutral order here is:

• Mój plecak jest cięższy niż twoja torba.

But other orders are possible depending on emphasis, for example:

• Cięższy jest mój plecak niż twoja torba.
• Mój plecak cięższy jest niż twoja torba.

These are less neutral and may sound more literary, emphatic, or marked.

For learners, the original version is the most natural and safest.

Twój / twoja / twoje is the informal singular your, used when speaking to one person whom you address as ty.

So here:

• twoja torba = your bag (one person, informal)

If you are speaking formally to one person, or to more than one person, you usually use forms of pański / pani / państwa or structures based on wasz depending on the situation.

For example:

• wasza torba = your bag (plural / informal to more than one person)

In this sentence, both plecak and torba are in the nominative singular.

• mój plecak
• twoja torba

That is because:

• plecak is the subject of the sentence
• after niż, Polish can keep the compared noun phrase in its normal nominative form

But if you used od, then torba would change to the genitive:

• Mój plecak jest cięższy od twojej torby.

So this sentence is useful because it shows a common comparison pattern with no case change after niż.

It can look difficult because of the consonant cluster.

A rough English-friendly guide is:

• cięższy ≈ CHEN-shshih (very approximate)

A few important points:

• ci before a vowel-like sound here gives a soft ć sound, similar to a soft ch
• ę is a nasal vowel
• ższ creates a dense sound cluster that learners often simplify at first

You do not need a perfect English spelling equivalent, because none is exact. It is better to listen to native audio and repeat slowly:

• cięż-
• -szy

Then combine them.

No. Those are wrong because the possessive must agree with the noun’s gender.

Correct forms are:

• mój plecak because plecak is masculine
• twoja torba because torba is feminine

Compare:

• mój plecak
• moja torba
• moje krzesło

This agreement is one of the most important things to learn in Polish.

The dictionary forms are:

• mój = my
• plecak = backpack
• być = to be
  
  • here it appears as jest = is
• ciężki = heavy
• niż = than
• twój = your
• torba = bag

This helps because some words in the sentence are not in the exact form you would find first in a dictionary:

• jest comes from być
• cięższy comes from ciężki
• twoja comes from twój

They are usually included if you want to clearly say my backpack and your bag.

Sometimes Polish omits possessives when ownership is obvious from context, but here they are important because the sentence is comparing my item with your item.

So the full sentence sounds natural and clear:

• Mój plecak jest cięższy niż twoja torba.

If you removed them:

• Plecak jest cięższy niż torba.

That would mean something more like:

• The backpack is heavier than the bag.

So the possessives matter a lot here.