Treść tego raportu jest jasna.

A word-by-word breakdown is:

• treść = content
• tego = of this
• raportu = report
• jest = is
• jasna = clear

So the structure is literally:

Content of-this report is clear.

Natural English: The content of this report is clear.

Because after treść in this meaning, Polish uses the genitive case to express of something.

• raport = report (base form, nominative)
• raportu = of the report (genitive singular)

So:

• treść raportu = the content of the report
• treść tego raportu = the content of this report

This is a very common pattern in Polish:

• autor książki = the author of the book
• kolor samochodu = the color of the car
• koniec filmu = the end of the film

Because tego has to match the case of raportu.

Here, raportu is in the genitive singular, so the demonstrative this must also be in the genitive singular:

• ten raport = this report (nominative)
• tego raportu = of this report (genitive)

So:

• Ten raport jest jasny. = This report is clear.
• Treść tego raportu jest jasna. = The content of this report is clear.

Those are different structures.

Because jasna agrees with treść, not with raportu.

The noun treść is:

• feminine
• singular
• nominative

So the adjective must also be:

• feminine
• singular
• nominative

That gives:

• jasny = masculine
• jasna = feminine
• jasne = neuter

Since treść is feminine, we say:

• Treść jest jasna.

Even though raport is masculine, it does not control the adjective here. The subject is treść.

The subject is treść.

The whole phrase treść tego raportu means the content of this report, but the main noun is treść. That is why:

• the verb is jest
• the adjective is jasna

Both relate to treść.

You can think of it like this:

• main noun: treść
• extra information: tego raportu = of this report

In standard Polish, yes.

Polish normally keeps the present-tense form of to be in sentences like this:

• Treść raportu jest jasna.

Unlike in some other Slavic languages, you usually do not omit jest in neutral standard Polish.

You may sometimes hear shortened or headline-style language without it, but for a learner, the safe standard form is:

• X jest Y

So here:

• Treść tego raportu jest jasna.

Yes, Polish word order is fairly flexible, although the neutral order here is very natural:

• Treść tego raportu jest jasna.

You could also hear:

• Jasna jest treść tego raportu.
• Treść raportu tego jest jasna. — possible, but less natural
• Tego raportu treść jest jasna. — marked, literary, or contrastive

For learners, the best default is the original sentence. It sounds natural and clear.

Because Polish does not have articles like a or the.

So Polish often expresses definiteness through context rather than a separate word.

• raport can mean a report or the report
• treść raportu can mean content of a report or the content of the report
• treść tego raportu clearly means the content of this report, because tego makes it specific

English needs the, but Polish does not.

Here it means clear in the sense of easy to understand.

The adjective jasny / jasna / jasne can mean different things depending on context:

• jasny pokój = a bright room
• jasny kolor = a light color
• jasna odpowiedź = a clear answer
• sprawa jest jasna = the matter is clear

In Treść tego raportu jest jasna, the meaning is clearly understandable / clear, not bright.

You could, but it means something slightly different.

• Treść tego raportu jest jasna. = The content of this report is clear.
• Ten raport jest jasny. = This report is clear.

The second sentence is less specific. It treats the whole report as clear. The original sentence focuses specifically on the content.

In many situations, both are possible, but they are not exactly the same.

A few helpful pronunciation notes:

• tr in treść is pronounced clearly, with both consonants sounded.
• ę in treść is a nasal vowel, but before ść it is often pronounced closer to eń or a nasalized e in normal speech.
• ść sounds roughly like a soft shch.
• raportu is pronounced with a rolled or tapped Polish r.
• Stress in Polish is usually on the second-to-last syllable:
  • RA-por-tu
  • JAS-na

A rough learner-friendly approximation might be:

• treść ≈ treshch
• raportu ≈ ra-POR-too
• jasna ≈ YAS-na

These are only approximations, but they can help at first.

Treść is in the nominative singular.

That is because it is the subject of the sentence:

• Treść tego raportu = subject
• jest = verb
• jasna = description of the subject

So the structure is:

• treść — nominative
• tego raportu — genitive phrase modifying treść
• jasna — nominative feminine singular, agreeing with treść

This is a very common pattern in Polish sentences with to be plus an adjective.