Ten segregator jest cięższy niż tamten, bo leżą w nim stare raporty.

Because segregator is a masculine singular noun in Polish.

• ten = this one, for masculine singular nouns
• tamten = that one, also masculine singular

So:

• ten segregator = this binder/file
• tamten (segregator) = that one / that binder

Polish demonstratives have to agree with the noun’s gender, number, and case.

Polish often leaves out a noun when it is obvious from context, just like English does.

So:

• Ten segregator jest cięższy niż tamten
  literally: This binder is heavier than that one

Here, tamten stands for tamten segregator. Repeating the noun would also be possible, but it is less natural if the meaning is already clear.

Cięższy is the comparative form of ciężki:

• ciężki = heavy
• cięższy = heavier

In Polish, comparatives are often formed with endings like -szy or -ejszy, and the stem may change a little. So this is not just a simple add-on in the way English adds -er.

A useful pair to remember is:

• ciężki = heavy
• cięższy = heavier

Niż means than in comparisons.

So:

• cięższy niż tamten = heavier than that one

It is the normal word to use after a comparative adjective like:

• większy niż... = bigger than...
• lepszy niż... = better than...
• cięższy niż... = heavier than...

Yes. Polish often allows both patterns:

• Ten segregator jest cięższy niż tamten.
• Ten segregator jest cięższy od tamtego.

Both mean the same thing.

The difference is grammatical:

• after niż, you get tamten
• after od, you use the genitive form: tamtego

So this is a very useful comparison to remember:

• niż tamten
• od tamtego

Nim is the pronoun form meaning in it / in him, depending on context. Here it clearly means in it, referring to segregator.

The preposition w normally takes the locative case when it means in:

• w segregatorze = in the binder
• w nim = in it

So w nim is the natural pronoun replacement for w segregatorze.

Leżą comes from leżeć, meaning to lie or to be lying.

Polish often uses leżeć for objects that are physically resting somewhere:

• Książki leżą na stole. = The books are lying on the table.
• Raporty leżą w nim. = The reports are lying in it.

You could say są w nim stare raporty in some contexts, but leżą gives a more physical, concrete image: the reports are actually sitting inside the binder.

Because the subject is stare raporty, which is plural.

• raport = report
• raporty = reports

So the verb must also be plural:

• raport leży = the report is lying
• raporty leżą = the reports are lying

The verb agrees with the subject.

Because adjectives in Polish must agree with the noun.

Here:

• raporty is plural
• it is masculine non-personal
• so the adjective takes the matching plural form stare

That gives:

• stare raporty = old reports

Compare:

• stary raport = old report
• stare raporty = old reports

Yes, Polish word order is quite flexible.

This sentence uses a natural order:

• bo leżą w nim stare raporty

But you could also hear:

• bo stare raporty leżą w nim

Both are grammatical. The difference is mostly about focus and style, not basic meaning.

The version in your sentence sounds natural and smooth, especially in everyday speech.

Bo means because.

It is very common in everyday Polish and sounds natural in speech.

You could also use more formal alternatives such as:

• ponieważ
• gdyż

So these are similar in meaning:

• bo leżą w nim stare raporty
• ponieważ leżą w nim stare raporty

But bo is the most conversational and common choice.

Polish does not have articles like English a and the.

So:

• segregator can mean a binder or the binder
• the exact meaning depends on context

In this sentence, ten and tamten already make the reference specific:

• ten segregator = this binder
• tamten = that one

So no article is needed.