Ta jest tańsza niż tamta.

Because Polish is using demonstrative pronouns here:

• ta = this one / that one (feminine, depending on context)
• tamta = that one over there / that other one (feminine)

In this sentence, they stand in for an omitted feminine noun, such as sukienka (dress) or książka (book).

So the sentence is structurally like:

• Ta [rzecz] jest tańsza niż tamta [rzecz].
• This [thing] is cheaper than that [thing].

Because the omitted noun is understood to be feminine singular.

Polish words change form to match the gender and number of the noun they refer to. So if the hidden noun is feminine, the pronouns must also be feminine.

For example:

• ta ... tamta → feminine singular
• ten ... tamten → masculine singular
• to ... tamto → neuter singular

So if you were talking about:

• ta sukienka → Ta jest tańsza niż tamta.
• ten telefon → Ten jest tańszy niż tamten.
• to krzesło → To jest tańsze niż tamto.

Because tańsza is the feminine singular form of the comparative adjective.

The base adjective is:

• tani = cheap

Its comparative is:

• tańszy = cheaper (masculine)
• tańsza = cheaper (feminine)
• tańsze = cheaper (neuter)

Since ta and tamta are feminine singular, the adjective must agree with them:

• Ta jest tańsza niż tamta.

It is the comparative form of tani.

Polish often forms comparatives by changing the adjective stem and adding a comparative ending. Here:

• tani → tańszy / tańsza / tańsze

This is not a completely transparent spelling change for a beginner, so it is best learned as a comparative pattern:

• tani = cheap
• tańszy = cheaper

Then the ending changes for gender:

• tańszy (masculine)
• tańsza (feminine)
• tańsze (neuter)

Niż means than in comparisons.

So:

• tańsza niż tamta = cheaper than that one

It is the standard word used after a comparative adjective such as:

• większy niż = bigger than
• ładniejsza niż = prettier than
• droższe niż = more expensive than

In this sentence, tamta stays in the nominative, because it stands for the full comparison:

• Ta jest tańsza niż tamta jest.

The second jest is omitted, but understood.

So tamta is still functioning like the subject of the omitted clause, which is why nominative makes sense here.

This is a very natural pattern in Polish.

Yes. That is also correct.

Polish has two common ways to say cheaper than that one:

• Ta jest tańsza niż tamta.
• Ta jest tańsza od tamtej.

The difference is grammatical:

• after niż, you get tamta
• after od, you use the genitive: tamtej

So both are correct, but the form changes depending on the comparison word.

Yes, in a normal full sentence, jest is expected.

• Ta jest tańsza niż tamta. = correct standard sentence

Leaving it out would sound incomplete or unnatural in ordinary speech:

• Ta tańsza niż tamta → not standard as a normal sentence

So for learners, it is best to keep jest.

No specific noun is written, but one is clearly understood from context.

Polish often omits a noun when it is obvious. For example:

• Ta sukienka jest tańsza niż tamta sukienka.
• shortened to:
• Ta jest tańsza niż tamta.

English does this too:

• This dress is cheaper than that dress.
• This one is cheaper than that one.

So the sentence is perfectly natural even without the noun.

Yes, depending on context and teaching style, but in contrast with tamta, it is usually best understood as this one or the one here.

A useful beginner contrast is:

• ta = this one
• tamta = that one / that one over there

Polish does not always map perfectly onto English this/that in every situation, but that contrast is a good practical guide here.

The neutral word order is:

• Ta jest tańsza niż tamta.

That is:

1. demonstrative pronoun (ta)
2. verb (jest)
3. comparative adjective (tańsza)
4. comparison phrase (niż tamta)

Polish word order is flexible, but this version is the most straightforward and natural for a learner.

Yes. The pronouns and adjective would all have to agree with the gender.

Examples:

• Ten jest tańszy niż tamten.
  masculine singular

• To jest tańsze niż tamto.
  neuter singular

So in Polish, agreement is very important: pronoun + verb context + adjective all match the implied noun.

Yes. Ta and tamta point to specific things already known in the situation.

So this is not a general statement like Things are getting cheaper. It is a comparison between two particular items:

• this one
• that one

That is why the sentence uses demonstratives instead of a general noun.