Moja siostra mówi, że ta sukienka jest ładna.

Because moja has to agree with siostra (sister), not with the speaker.

In Polish, possessive words like my change their form depending on the gender, number, and case of the noun they describe:

• mój = masculine singular
• moja = feminine singular
• moje = neuter singular

Since siostra is feminine singular, you use moja.

Siostra is in the nominative singular, which is the basic dictionary form.

It is nominative because it is the subject of the sentence:

• Moja siostra mówi... = My sister says...

A lot of feminine nouns in Polish end in -a in the nominative singular, and siostra is a typical example.

It can mean either, depending on context.

Mówi is the 3rd person singular present tense of mówić. In English, Polish present tense can often correspond to:

• says
• is saying
• speaks
• is speaking

In this sentence, the most natural meaning is says:

• Moja siostra mówi, że... = My sister says that...

So Polish does not force the same distinction that English often makes between says and is saying.

Because in Polish, a comma is normally used before że when it introduces a subordinate clause.

So:

• Moja siostra mówi, że...

This is standard Polish punctuation. It is different from English, where you usually do not write a comma before that in a sentence like My sister says that...

Że introduces a subordinate clause and usually means that.

So the sentence breaks down like this:

• Moja siostra mówi = My sister says
• że ta sukienka jest ładna = that this dress is pretty

It is a very common word after verbs of saying, knowing, thinking, etc.

Examples:

• Wiem, że... = I know that...
• Myślę, że... = I think that...
• Mówi, że... = He/She says that...

Because ta has to agree with sukienka.

Sukienka (dress) is:

• feminine
• singular
• nominative

So the demonstrative must also be feminine singular nominative:

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

That is why you get:

• ta sukienka = this dress

Not always strictly. In many situations, Polish ten / ta / to can cover both this and that, depending on context.

So ta sukienka often means this dress, but in some contexts English might translate it as that dress.

Polish does not always make the same distance distinction as English.

Here, jest is needed.

Jest is the 3rd person singular of być (to be), and it links the subject with the adjective:

• ta sukienka jest ładna = this dress is pretty

In modern Polish, you normally keep jest in this kind of sentence.

So you say:

• Sukienka jest ładna.

not normally:

• Sukienka ładna.
  except in very informal, poetic, or headline-style language.

Because adjectives in Polish must agree with the noun they describe.

Sukienka is feminine singular, so the adjective must also be feminine singular:

• ładny = masculine
• ładna = feminine
• ładne = neuter

So:

• ładna sukienka = a pretty dress
• ta sukienka jest ładna = this dress is pretty

Even though ładna comes after jest, it still agrees with sukienka.

Because Polish does not have articles like English a/an and the.

So sukienka can mean:

• a dress
• the dress
• sometimes just dress

The exact meaning depends on context.

In this sentence, ta sukienka makes it definite anyway, because ta points to a specific dress:

• ta sukienka = this dress

Yes, Polish word order is more flexible than English, because endings carry a lot of grammatical information.

The neutral order here is:

• Moja siostra mówi, że ta sukienka jest ładna.

But other orders are possible for emphasis, for example:

• Moja siostra mówi, że ładna jest ta sukienka.
• Ta sukienka jest ładna, mówi moja siostra.

These variants may sound more marked, emphatic, or stylistically different. For a learner, the original version is the safest and most natural basic pattern.

Że is pronounced roughly like zheh.

More precisely:

• ż sounds like the s in measure or the g in genre
• e is like e in bet

So że sounds approximately like zhe

It is not pronounced like English gee or zee.

A few useful ones here are:

• ł in ładna sounds like English w
  • so ładna starts roughly like wad-
• ó in mówi sounds like u in Polish, roughly like English oo
• ż in że is the zh sound, like in measure

Very rough pronunciation of the whole sentence:

• MO-ya SHO-stra MOO-vee, zhe ta soo-KYEN-ka yest WAD-na

That is only an approximation, but it is a helpful starting point.