Ten materac jest twardszy niż tamten, ale śpi mi się na nim lepiej.

They are demonstratives:

• ten materac = this mattress
• tamten = that one / that mattress

Both words agree with materac in gender, number, and case. Since materac is masculine singular nominative here, we get ten and tamten.

In English, that one often replaces a repeated noun, and Polish does the same here: tamten stands for tamten materac.

Because materac is a masculine noun in Polish. Most inanimate nouns ending in a consonant are masculine, and materac fits that pattern.

That is why the words around it also take masculine forms:

• ten materac
• materac jest twardszy
• tamten

If the noun were feminine or neuter, these forms would change.

Twardy means hard / firm.
Twardszy is the comparative form: harder / firmer.

So:

• twardy = hard
• twardszy = harder

Polish often makes comparatives with special endings rather than using a separate word like English more. So here Polish uses a built-in comparative form.

Usually, no. Twardszy is the normal, natural comparative.

Polish can use bardziej + adjective, but that is mainly for adjectives that do not form a simple comparative easily, or when the speaker wants a special stylistic effect. With a common adjective like twardy, twardszy is what learners should use.

Niż means than in comparisons.

So:

• twardszy niż tamten = harder than that one

It introduces the second thing being compared.

Yes. That is another normal way to say it.

You can compare in two common ways:

• twardszy niż tamten
• twardszy od tamtego

The main difference for a learner is the form after it:

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

Both are natural.

Because Polish, like English, often omits a repeated noun when it is obvious.

So:

• Ten materac jest twardszy niż tamten
• literally: This mattress is harder than that one

The full version niż tamten materac is possible, but it sounds more repetitive.

This is a very common Polish pattern:

verb + dative pronoun + się

It often expresses how something feels or goes for someone, rather than presenting the action in a plain, direct way.

So śpi mi się is not a literal reflexive I sleep myself. It is more like:

• I sleep
• it is comfortable/easy for me to sleep
• I find sleeping ...

In this sentence, it gives the idea of personal sleeping experience on the mattress.

Mi is the dative form of I / me. In this structure, it marks the experiencer: the person for whom the action feels a certain way.

So:

• śpi mi się dobrze = I sleep well / it is comfortable for me to sleep
• śpi mi się lepiej = I sleep better

A rough literal breakdown would be something like it sleeps better to me, but that is only to help you see the grammar. In real English, you would translate it naturally.

Correct: here się is not truly reflexive in the sense of wash oneself.

In sentences like śpi mi się dobrze, pracuje mi się dobrze, czyta mi się łatwo, się helps create an impersonal, experiential construction. It shifts the meaning away from a simple direct statement and toward how it feels to do something.

Compare:

• Śpię na nim lepiej = I sleep better on it
  More direct, plain statement.

• Śpi mi się na nim lepiej = I sleep better on it / It’s easier or more comfortable for me to sleep on it
  More experiential and idiomatic in this context.

Nim is the form of on/it/he used after certain prepositions. Here it is the pronoun form of on after na in the locative case.

Why locative? Because na here means location: on it.

• śpię na nim = I sleep on it

If you were talking about motion onto something, Polish would use a different case:

• położyć coś na niego / na materac type patterns involve movement

But here there is no movement; it is just location, so na nim is correct.

Because lepiej is an adverb, and it describes how someone sleeps.

• lepszy = better as an adjective, used with nouns
• lepiej = better as an adverb, used with verbs

Here the word modifies śpi się:

• śpi się lepiej = one sleeps better

If you said lepszy materac, that would mean a better mattress. But in this sentence, the point is not directly that the mattress is better; it is that the sleeping experience is better.

Yes. That is grammatical and natural.

But there is a nuance:

• Śpię na nim lepiej = straightforward, direct
• Śpi mi się na nim lepiej = a bit softer, more idiomatic when talking about comfort or personal experience

The version with mi się is especially common when describing how easy, pleasant, comfortable, or difficult an activity feels.

No, Polish word order is fairly flexible.

The given sentence is natural:

• Ten materac jest twardszy niż tamten, ale śpi mi się na nim lepiej.

But other orders are possible, for example:

• ..., ale lepiej mi się na nim śpi.

That version puts extra focus on lepiej.

So the basic meaning stays the same, but word order can shift emphasis.

Because ale means but, and in Polish it normally introduces a new clause. Standard punctuation puts a comma before it.

So:

• ..., ale ... = ..., but ...

This works very much like English punctuation in a sentence of this kind.