Mój sweter jest grubszy niż twoja bluza.

Because Polish possessive words like mój and twój must agree with the noun they describe in gender, number, and case.

• sweter is masculine singular, so you use mój
• bluza is feminine singular, so you use twoja

That is why the sentence has:

• mój sweter
• twoja bluza

The same agreement happens with adjectives too.

Both are in the nominative in this sentence.

• Mój sweter is the subject, so nominative is expected.
• After niż, Polish commonly keeps the compared noun phrase in the nominative too: niż twoja bluza.

A very useful comparison:

• Mój sweter jest grubszy niż twoja bluza.
• Mój sweter jest grubszy od twojej bluzy.

After od, you need the genitive, so twoja bluza changes to twojej bluzy.

Grubszy is the comparative form of gruby.

• gruby = thick
• grubszy = thicker

Polish often forms comparatives with endings like -szy or -ejszy, though the stem sometimes changes a little. Here:

• gruby → grubszy

This is just the normal comparative form you need here, much like English thick → thicker.

Because it agrees with sweter, which is masculine singular.

Even though grubszy comes after jest, it still has to match the noun it describes.

Here are the matching forms:

• masculine: grubszy
• feminine: grubsza
• neuter: grubsze

So with sweter you need grubszy.

Usually, no. The normal and natural form is grubszy.

In Polish, many common adjectives prefer a built-in comparative form rather than bardziej + adjective.

So:

• natural: grubszy
• less natural here: bardziej gruby

Bardziej is more commonly used when there is no simple comparative form, or in certain stylistic contexts.

In a normal sentence like this, jest is needed.

Polish often drops subject pronouns such as ja, ty, on, but it does not normally drop the verb to be in this kind of present-tense sentence.

So:

• correct: Mój sweter jest grubszy niż twoja bluza.
• without jest: possible only in very informal, poetic, or elliptical styles, but not the standard choice for learners

For regular speech and writing, keep jest.

Yes, you can often use either niż or od after a comparative.

Both can express than in sentences like this:

• Mój sweter jest grubszy niż twoja bluza.
• Mój sweter jest grubszy od twojej bluzy.

The main difference is grammatical:

• after niż: usually nominative phrase
• after od: genitive is required

So the sentence changes from twoja bluza to twojej bluzy if you use od.

Because niż does not force the genitive the way od does.

With niż, Polish usually keeps the second phrase in a form that matches a full comparison, as if the sentence were something like:

• Mój sweter jest grubszy niż twoja bluza jest gruba.

That full version is not how people normally say it, but it helps explain why twoja bluza stays in the nominative.

By contrast:

• od twojej bluzy requires genitive because od governs that case.

Yes, gruby can mean fat when talking about a person, but with objects it usually means thick.

So in this sentence:

• grubszy sweter = a thicker sweater

That is completely natural. For clothing, grubszy usually suggests thicker material, and often by implication something warmer or heavier too.

So the meaning depends on the noun:

• gruby człowiek = a fat person
• gruby sweter = a thick sweater
• gruba książka = a thick book

Not completely. Polish word order is more flexible than English because endings carry a lot of the grammatical information.

The version you have is the most neutral and natural:

• Mój sweter jest grubszy niż twoja bluza.

You can change the order for emphasis, but the sentence may sound marked or literary. For example, moving mój sweter or grubszy to the front can emphasize contrast.

For learners, the safest choice is to keep the neutral order used here.

Because Polish has no articles.

English uses a and the, but Polish does not. Whether something is definite or indefinite is understood from context.

So mój sweter naturally means my sweater, without needing any separate word for the.

That is normal in Polish, and learners just have to get used to not translating articles directly.