Ta owsianka jest cieplejsza niż jogurt, dlatego zimą jem ją częściej.

Because owsianka is a feminine singular noun, and the demonstrative this must agree with it in gender, number, and case.

So:

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

That is why we say ta owsianka.

Owsianka is the subject of the sentence, so it is in the nominative singular.

The sentence is saying that the oatmeal is warmer, so owsianka is the thing doing the being in jest cieplejsza.

Because in standard Polish, when you say X is Y, you normally use the verb być in the present tense.

So:

• Ta owsianka jest cieplejsza... = This oatmeal is warmer...

Unlike in some languages, you usually cannot just leave out is here in normal Polish.

Cieplejsza is the comparative form of ciepła / ciepły, meaning warmer.

Base form:

• ciepły = warm

Comparative:

• cieplejszy = warmer

Because it describes owsianka, which is feminine singular, it has to agree:

• masculine: cieplejszy
• feminine: cieplejsza
• neuter: cieplejsze

So cieplejsza is used because owsianka is feminine.

Niż means than in comparisons.

So:

• cieplejsza niż jogurt = warmer than yogurt

It is the normal word used after a comparative adjective or adverb when you compare two things.

Because this sentence uses the comparison pattern with niż.

Compare:

• cieplejsza niż jogurt = warmer than yogurt
• cieplejsza od jogurtu = warmer than yogurt

Both are possible, but they use different structures:

• niż + jogurt
• od + genitive, so jogurtu

So jogurt is correct here because the sentence uses niż.

Dlatego means therefore, because of that, or that’s why.

It introduces the result of the first clause:

• Ta owsianka jest cieplejsza niż jogurt
• dlatego zimą jem ją częściej

So the logic is: It is warmer than yogurt, so in winter I eat it more often.

Because the sentence has two clauses, and dlatego introduces the second one.

The comma separates:

1. Ta owsianka jest cieplejsza niż jogurt
2. dlatego zimą jem ją częściej

This is normal Polish punctuation.

Because Polish often uses the instrumental case with seasons to mean in a given season.

So:

• zima = winter
• zimą = in winter / during winter

This is a very common pattern:

• latem = in summer
• jesienią = in autumn
• wiosną = in spring
• zimą = in winter

You can also hear w zimie, but zimą is very natural and common.

Because Polish usually omits subject pronouns when they are clear from the verb ending.

• jem already means I eat

So ja is unnecessary unless you want extra emphasis or contrast.

For example:

• Jem ją częściej = I eat it more often
• Ja jem ją częściej = I eat it more often

The second version sounds more emphatic.

Ją is the accusative singular feminine form of ona, and here it means it.

It refers back to owsianka, which is a feminine noun.

Because jeść takes a direct object, Polish uses the accusative:

• jem ją = I eat it

So:

• ona = she / it
• ją = her / it as a direct object

Because częściej is the normal comparative adverb meaning more often.

Base form:

• często = often

Comparative:

• częściej = more often

It is an adverb, not an adjective, because it describes how often the action jem happens.

So:

• jem ją częściej = I eat it more often

Using bardziej często would sound unnatural in normal Polish.

No, Polish word order is fairly flexible, but different orders change the focus or emphasis.

The version in the sentence is natural and neutral:

• dlatego zimą jem ją częściej

Other possible orders:

• dlatego częściej jem ją zimą
• dlatego ją jem częściej zimą

These are possible, but they may sound less neutral or put emphasis in a different place.

So the original order is a good one to learn first.

Yes. A very natural alternative is:

Ta owsianka jest cieplejsza od jogurtu, dlatego w zimie jem ją częściej.

This changes two things:

• niż jogurt → od jogurtu
• zimą → w zimie

Both versions are natural. The original sentence is perfectly correct, and this alternative is also good Polish.