Ten akapit jest krótki, ale treść całego rozdziału wydaje mi się bardzo trudna.

Ten means this.

So ten akapit = this paragraph.

Polish demonstratives change depending on the gender and case of the noun:

• ten for masculine nouns
• ta for feminine nouns
• to for neuter nouns

Because akapit is a masculine noun, you use ten.

Examples:

• ten akapit = this paragraph
• ta książka = this book
• to zdanie = this sentence

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

Here, krótki describes akapit, and akapit is:

• masculine
• singular
• nominative

So the adjective also has to be masculine singular nominative:

• krótki

Compare:

• ten akapit jest krótki = this paragraph is short
• ta książka jest krótka = this book is short
• to opowiadanie jest krótkie = this story is short

In standard Polish, no. You normally keep jest in the present tense when saying X is Y.

So:

• Ten akapit jest krótki = correct
• Ten akapit krótki = not standard as a normal sentence

Polish often omits to be in some other contexts less than Russian, and in headlines or informal notes you may see shortened styles, but in a normal sentence like this, jest should stay.

This is a very useful vocabulary question.

• akapit = paragraph in normal writing
• paragraf usually means section, especially in laws, contracts, or formal documents

So in this sentence, akapit is definitely the right word for a paragraph of text.

Yes, ale means but here.

It connects two contrasting ideas:

• This paragraph is short
• but the content of the whole chapter seems very difficult to me

So ale works just like English but in this sentence.

Treść is the subject of the second clause, so it is in the nominative:

• treść = content

Then całego rozdziału means of the whole chapter, and that part is in the genitive because it depends on treść.

So the structure is:

• treść = content
• całego rozdziału = of the whole chapter

Literally:

• content of the whole chapter

This is a very common Polish pattern:

• treść książki = the content of the book
• koniec filmu = the end of the film
• autor artykułu = the author of the article

Because całego describes rozdziału, and adjectives must agree with the nouns they modify.

Here:

• rozdział = chapter
• after treść, we need of the chapter, so rozdział becomes genitive singular: rozdziału
• the adjective cały must match it, so it becomes całego

So:

• nominative: cały rozdział = the whole chapter
• genitive: całego rozdziału = of the whole chapter

Wydaje mi się is a very common Polish way to say it seems to me or I find / I feel / it strikes me as.

Literally:

• wydaje się = seems
• mi = to me

So:

• treść całego rozdziału wydaje mi się bardzo trudna literally means:
• the content of the whole chapter seems very difficult to me

This is one of the most natural ways in Polish to express a personal impression without sounding too direct.

Compare:

• To wydaje mi się dziwne. = That seems strange to me.
• Ona wydaje mi się miła. = She seems nice to me.

Both can mean to me, but mi is the usual unstressed form, and mnie is used for emphasis or after some prepositions.

So in a neutral sentence:

• wydaje mi się = the normal choice

If you wanted emphasis, you could say:

• mnie się wydaje = I think / to me it seems

Very roughly:

• mi = ordinary, unstressed
• mnie = stronger, more emphatic

Because trudna describes treść, and treść is a feminine noun.

So the adjective must match:

• treść = feminine singular nominative
• trudna = feminine singular nominative

Compare:

• ten tekst jest trudny = this text is difficult
• ta treść jest trudna = this content is difficult
• to zadanie jest trudne = this task is difficult

Bardzo trudna means very difficult.

Yes, bardzo usually goes before the adjective or adverb it modifies:

• bardzo trudna = very difficult
• bardzo krótki would not work here because the adjective form is wrong for akapit, but bardzo krótki akapit = a very short paragraph

You can also use bardzo with verbs in some contexts:

• bardzo lubię = I like very much

In this sentence, bardzo naturally comes before trudna.

The given word order is natural and neutral:

• Ten akapit jest krótki, ale treść całego rozdziału wydaje mi się bardzo trudna.

Polish word order is more flexible than English because endings show grammatical relationships, but changing the order changes emphasis.

For example:

• Treść całego rozdziału wydaje mi się bardzo trudna. = neutral
• Bardzo trudna wydaje mi się treść całego rozdziału. = more literary or emphatic
• Mnie się wydaje, że treść całego rozdziału jest bardzo trudna. = also natural, slightly different structure

So the original order is probably the best one for a learner to use first.

Grammatically, people would understand it, but wydaje mi się is the normal and idiomatic order.

In Polish, short unstressed pronouns like mi, ci, mu often appear early in the clause, and wydaje mi się is the standard phrase you should learn as a chunk.

So:

• wydaje mi się = natural
• wydaje się mi = much less natural

There is a word for the-like reference in Polish only indirectly; Polish has no articles like the or a.

So:

• treść całego rozdziału can mean the content of the whole chapter
• the context tells you it is definite

This is normal in Polish:

• Książka jest ciekawa. = The book is interesting / A book is interesting, depending on context
• Treść rozdziału jest trudna. = The content of the chapter is difficult

English uses articles; Polish does not.

A more literal version would be:

This paragraph is short, but the content of the whole chapter seems to me very difficult.

That is not the most natural English, but it helps show the Polish structure:

• Ten akapit = this paragraph
• jest krótki = is short
• ale = but
• treść całego rozdziału = the content of the whole chapter
• wydaje mi się = seems to me
• bardzo trudna = very difficult

A more natural English translation would usually be: This paragraph is short, but the content of the whole chapter seems very difficult to me.