Ten raport ma dwadzieścia cztery strony, ale najważniejszy slajd pokazałam dopiero na końcu.

Because raport is a masculine singular noun, and ten is the masculine singular nominative form of this.

So:

• ten raport = this report
• ta would be for a feminine noun
• to would be for a neuter noun

Here ten raport is the subject of the first clause, so it is in the nominative case.

That is a very normal Polish way to talk about length or size in pages.

• Ten raport ma dwadzieścia cztery strony literally = This report has twenty-four pages
• In natural English, we often say This report is twenty-four pages long
• In Polish, using mieć (to have) is standard here

So even though the literal wording is different from English, the structure is completely natural in Polish.

Because Polish number constructions change depending on the final digit.

With non-masculine-personal nouns like strona:

• after numbers ending in 2, 3, 4 (except 12, 13, 14) you usually use the plural form like strony
• after 5 and higher numbers of the other type, you often use the genitive plural, here stron

So:

• dwie strony = two pages
• cztery strony = four pages
• dwadzieścia cztery strony = twenty-four pages
• pięć stron = five pages
• dwadzieścia pięć stron = twenty-five pages

In this sentence, strony is the correct form.

It is the direct object of ma, so syntactically it is in the accusative plural.

For the noun strona, the accusative plural looks the same as the nominative plural:

• nominative plural: strony
• accusative plural: strony

So the form is strony, even though its job in the sentence is object, not subject.

Because slajd is a masculine singular noun, and the adjective has to agree with it in gender, number, and case.

• ważny = important
• ważniejszy = more important
• najważniejszy = most important

So:

• najważniejszy slajd = the most important slide

Here the phrase is masculine singular because slajd is masculine singular.

It is actually the direct object of pokazałam, so it is in the accusative singular.

However, with masculine inanimate nouns like slajd, the accusative singular is the same as the nominative singular. That is why it looks like a subject form.

So:

• nominative: najważniejszy slajd
• accusative: najważniejszy slajd

Same form, different function.

Because the speaker is female.

In the Polish past tense, the verb agrees with the gender of the speaker:

• pokazałam = I showed said by a woman
• pokazałem = I showed said by a man

So this sentence tells you that the person speaking is female.

This is about aspect.

• pokazać / pokazałam = perfective, a completed action
• pokazywać / pokazywałam = imperfective, ongoing, repeated, or habitual action

Here the speaker is talking about one completed event in a presentation: she showed the most important slide at the end. That is why perfective pokazałam is the natural choice.

If you used pokazywałam, it would suggest something more like I was showing or I used to show, which does not fit as well here.

Dopiero adds the idea of only then, not until then, or as late as.

So:

• pokazałam dopiero na końcu means something like
  I only showed it at the end
  or
  I didn’t show it until the end

It suggests that the important slide came later than one might expect.

Here na końcu is correct because it means at the end of something, such as a presentation, list, or sequence.

• na końcu = at the end
• w końcu usually means finally, eventually, or sometimes after all

So:

• pokazałam dopiero na końcu = I showed it only at the end
• w końcu pokazałam would mean more like I finally showed it

Also, końcu is the locative form after na in this expression.

Because in Polish, a comma before ale is normally required.

Ale means but, and Polish punctuation usually separates clauses joined by this kind of conjunction.

So:

• Ten raport ma dwadzieścia cztery strony, ale...

That comma is not optional in standard writing.

Yes. Polish word order is more flexible than English, and changing the order changes the emphasis.

The given version:

• Ten raport ma dwadzieścia cztery strony, ale najważniejszy slajd pokazałam dopiero na końcu.

puts the contrast first and then gives the key information that the crucial slide came only at the end.

Another natural version would be:

• Ten raport ma dwadzieścia cztery strony, ale dopiero na końcu pokazałam najważniejszy slajd.

This version gives a little more emphasis to dopiero na końcu.

Both are grammatical; the difference is mainly one of focus and style.