Ten sernik jest pyszny, chociaż trochę za słodki dla mojej siostry.

Because sernik is a masculine singular noun, and the demonstrative ten has to agree with it.

Polish this/that words change for gender:

• ten = masculine
• ta = feminine
• to = neuter

So:

• ten sernik = this cheesecake
• ta kawa = this coffee
• to ciasto = this cake

Even though English just uses this, Polish has to match the noun’s gender.

Yes. In Polish, grammatical gender is not the same as biological sex. Objects, foods, and ideas all have grammatical gender.

Sernik is masculine inanimate. A native speaker simply learns it as a masculine noun.

Compare:

• sernik = masculine
• kawa = feminine
• ciasto = neuter

A common clue is the ending:

• nouns ending in a consonant are often masculine
• nouns ending in -a are often feminine
• nouns ending in -o / -e are often neuter

So sernik ends in a consonant, which fits the masculine pattern.

Because adjectives in Polish must agree with the noun they describe in gender, number, and case.

Here the noun is sernik, which is:

• masculine
• singular
• nominative

So the adjectives also appear in masculine singular nominative:

• pyszny
• słodki

Compare:

• Ten sernik jest pyszny.
• Ta kawa jest pyszna.
• To ciasto jest pyszne.

The adjective changes depending on the noun.

With an adjective like pyszny, Polish normally uses the nominative after jest.

So:

• Ten sernik jest pyszny.
• Zupa jest gorąca.
• To ciasto jest dobre.

That is completely normal.

You may be thinking of cases where a noun comes after być. In those cases, Polish often uses the instrumental, for example:

• On jest lekarzem. = He is a doctor.

But with adjectives like pyszny or słodki, nominative is the standard form.

Za before an adjective means too in the sense of excessively.

So:

• słodki = sweet
• za słodki = too sweet

Other examples:

• za drogi = too expensive
• za trudny = too difficult
• za głośny = too loud

So the sentence is not just saying the cheesecake is sweet. It is saying that, for the sister, the sweetness goes a bit beyond what she likes.

Trochę means a little, a bit, or somewhat.

So:

• za słodki = too sweet
• trochę za słodki = a little too sweet / a bit too sweet

It softens the statement. Without trochę, the judgment sounds stronger.

Compare:

• Jest za słodki. = It’s too sweet.
• Jest trochę za słodki. = It’s a bit too sweet.

Because trochę modifies the whole idea too sweet.

So trochę za słodki means:

• a bit too sweet

That is the natural order in Polish.

A learner might be tempted to say something like za trochę słodki, but that is not natural here. The standard phrasing is:

• trochę za słodki
• trochę za drogi
• trochę za późno

Because chociaż introduces a subordinate clause meaning although / even though.

In standard Polish punctuation, subordinate clauses are usually separated by a comma, so:

• Ten sernik jest pyszny, chociaż trochę za słodki dla mojej siostry.

That comma is normal and expected in careful writing.

Chociaż means although, though, or even though. It introduces a contrast where both things are true at the same time.

Here the idea is:

• the cheesecake is delicious
• even so, it is a bit too sweet for the sister

You could use ale in some contexts:

• Ten sernik jest pyszny, ale trochę za słodki dla mojej siostry.

That also works, but the nuance is slightly different:

• ale = but
• chociaż = although / even though

Chociaż sounds a bit more like a concession: despite that, ...

It is omitted because it is easy to understand from the first part of the sentence.

The full version could be:

• Ten sernik jest pyszny, chociaż jest trochę za słodki dla mojej siostry.

That is grammatically correct too. But Polish often leaves out repeated words when the meaning is clear.

So the shorter version sounds natural and efficient:

• ..., chociaż trochę za słodki dla mojej siostry.

English does something similar sometimes:

• Delicious, although a bit too sweet for her.

Because the preposition dla always requires the genitive case.

The basic form is:

• moja siostra = my sister

After dla, it changes to genitive:

• dla mojej siostry = for my sister

Both words change:

• moja → mojej
• siostra → siostry

So dla moja siostra is incorrect because it leaves the noun phrase in the nominative instead of the genitive.

Mojej siostrze is the dative form, but after dla you must use the genitive, not the dative.

So:

• dla mojej siostry = correct
• dla mojej siostrze = incorrect

It is true that siostrze is a real Polish form, but it is used in different structures, for example:

• Daję prezent mojej siostrze. = I am giving a present to my sister.

In your sentence, the structure is specifically dla + genitive.

Here it means for my sister’s taste / from my sister’s point of view.

So the idea is:

• the cheesecake is delicious
• but according to my sister, it is a bit too sweet

Polish often uses dla + genitive this way:

• To jest za trudne dla dzieci. = This is too difficult for children.
• Ta muzyka jest za głośna dla mnie. = This music is too loud for me.

It does not necessarily mean the thing is intended as a gift or meant for someone physically. Very often it means in that person’s opinion or relative to that person.

Polish word order is fairly flexible, but the original sentence sounds neutral and natural:

• Ten sernik jest pyszny, chociaż trochę za słodki dla mojej siostry.

You can move parts around, but the emphasis changes. For example:

• Ten sernik jest pyszny, chociaż dla mojej siostry jest trochę za słodki.

This version puts a bit more emphasis on dla mojej siostry.

So yes, word order can change, but not all versions sound equally natural in every context. The original is a very good standard phrasing.

A few useful pronunciation notes:

• j sounds like English y
• ł sounds like English w
• sz sounds like sh
• ch is a hard h sound, like in Scottish loch
• ó sounds like oo

Rough guides:

• sernik = SER-neek
• jest = yest
• pyszny = roughly PISH-nih
• chociaż = roughly HO-chaż, with a soft middle consonant; the exact Polish sound is softer than English
• trochę = roughly TRO-heh, with hard ch
• słodki = roughly SWOD-kee
• siostry = roughly SHOS-trih, though the first sound is softer than English sh

Also remember that Polish stress is usually on the second-to-last syllable:

• SER-nik
• PYSZ-ny
• SŁOD-ki
• SIO-stry