Plama jest znacznie mniejsza.

Because plama is a feminine singular noun in Polish, and the adjective has to agree with it.

• plama = feminine singular
• so the adjective must also be feminine singular: mniejsza

Compare:

• mały stół → stół jest mniejszy
• mała plama → plama jest mniejsza
• małe okno → okno jest mniejsze

So mniejsza is the correct form because it matches plama.

Mniejsza is the comparative form of mała / mały / małe = small.

So:

• mała = small
• mniejsza = smaller

This is not formed regularly like English small → smaller by just adding one ending. In Polish, many comparatives are built with stem changes:

• mały → mniejszy / mniejsza / mniejsze
• duży → większy / większa / większe
• dobry → lepszy / lepsza / lepsze

So mniejsza means smaller, not just small.

Jest means is. It is the 3rd person singular form of być = to be.

So:

• plama = the stain / the spot
• jest = is
• znacznie mniejsza = much / considerably smaller

Yes, in a normal sentence you do need jest. Unlike in some other Slavic languages, Polish normally keeps the present-tense form of to be in sentences like this.

So:

• Plama jest mniejsza. = correct
• Plama mniejsza. = not a normal full sentence in standard Polish

You might see omission in headings, notes, or very informal contexts, but learners should use jest.

Znacznie means considerably, significantly, or often simply much in natural English.

It strengthens the comparative adjective:

• mniejsza = smaller
• znacznie mniejsza = much smaller / considerably smaller

It works like an adverb modifying the comparative.

Other common intensifiers with comparatives are:

• dużo mniejsza = much smaller
• o wiele mniejsza = much smaller / far smaller
• trochę mniejsza = a little smaller
• nieco mniejsza = slightly smaller

So znacznie adds emphasis: not just smaller, but noticeably or considerably smaller.

In Polish, the normal way to say smaller is mniejsza, not bardziej mała.

Polish often prefers a synthetic comparative (a special comparative form) instead of using bardziej + adjective.

So:

• mniejsza = natural and standard
• bardziej mała = unnatural in this context

You do use bardziej with adjectives that do not usually have a simple comparative form, or when you want a different stylistic effect. But with mały, the normal comparative is definitely mniejszy / mniejsza / mniejsze.

Yes, Polish word order is more flexible than English, but the basic neutral order here is very natural:

• Plama jest znacznie mniejsza.

That sounds like a normal statement: The stain is much smaller.

You can move words for emphasis, for example:

• Znacznie mniejsza jest plama.
  This is more marked and sounds like you are emphasizing much smaller.

• Plama jest mniejsza znacznie.
  Possible in some contexts, but less neutral and less common.

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

Plama most often means stain, spot, or blotch, depending on context.

Examples:

• plama na koszuli = a stain on a shirt
• plama atramentu = an ink stain
• ciemna plama = a dark spot / blotch

So in English, the best translation depends on the situation. The Polish grammar of the sentence does not change, but the most natural English word might.

Here plama is in the nominative singular, because it is the subject of the sentence.

The adjective mniejsza is also in the nominative singular feminine because it agrees with plama.

With być in a sentence like this, the adjective describing the subject is normally in the nominative:

• Plama jest mała.
• Plama jest mniejsza.

So this is not an object, and there is no special case change here beyond normal agreement.

A rough English-friendly guide could be:

• Plama ≈ PLA-ma
• jest ≈ yest
• znacznie ≈ ZNACH-nye
• mniejsza ≈ MNESH-sha

A few useful pronunciation notes:

• j in Polish is like English y
• cz sounds like ch in chop
• nie often sounds roughly like nye
• sz sounds like sh
• mn at the start of mniejsza can feel awkward for English speakers, but both consonants are pronounced

The stress in Polish is usually on the second-to-last syllable:

• PLA-ma
• jest
• ZNACZ-nie
• MNIEJ-sza

Yes. Polish does not have articles like the and a/an, so plama by itself can mean:

• the stain
• a stain
• sometimes even just stain in a more general sense

The exact English translation depends on context.

So:

• Plama jest znacznie mniejsza.
  could be The stain is much smaller.
• In another context, it might be A stain is much smaller.

Usually, context makes it clear which one is meant.

You would change both the noun and the adjective so they agree in number:

• singular: Plama jest znacznie mniejsza.
• plural: Plamy są znacznie mniejsze.

Changes:

• plama → plamy = stains
• jest → są = are
• mniejsza → mniejsze = smaller, plural form here

So the pattern is:

• feminine singular: mniejsza
• non-masculine-personal plural: mniejsze

Since plamy is not a masculine-personal noun, Polish uses mniejsze, not mniejsi.