Ten kocyk jest cieńszy niż tamten, ale niemowlę lubi go najbardziej.

They are demonstratives:

• ten = this
• tamten = that

They must agree with the noun they describe. Kocyk is masculine singular nominative, so you get:

• ten kocyk = this blanket
• tamten kocyk = that blanket

In the sentence, tamten stands on its own because kocyk is understood already:
Ten kocyk jest cieńszy niż tamten = This blanket is thinner than that one.

Kocyk is a diminutive of koc.

• koc = blanket
• kocyk = little blanket, small blanket, or a more affectionate word for blanket

In many contexts, especially with babies, kocyk sounds very natural and can suggest a baby blanket or simply a blanket spoken of warmly.

So this is not just grammar; it also gives the sentence a softer, more natural tone.

The basic adjective is:

• cienki = thin

Its comparative is:

• cieńszy = thinner

This is a normal Polish comparative pattern, although the stem changes a bit:

• cienki → cieńszy

You cannot predict every stem change perfectly at first, so it is best to learn common comparatives as vocabulary items.

Useful set:

• cienki = thin
• cieńszy = thinner
• najcieńszy = thinnest

Normally, no. For cienki, Polish strongly prefers the simple comparative:

• cieńszy = correct and natural

Using bardziej cienki would sound unnatural in standard Polish.

A good rule is:

• many common adjectives use a built-in comparative form, like większy, mniejszy, lepszy, cieńszy
• bardziej is used when there is no simple comparative, or when speakers want a different stylistic effect

So here, cieńszy is definitely the form to use.

Yes. Both can express than in comparisons.

This sentence has:

• cieńszy niż tamten = thinner than that one

A very common alternative is:

• cieńszy od tamtego

The main difference for a learner is the form that follows:

• after niż, you often get a form like tamten
• after od, you need the genitive, so tamtego

So both are correct:

• Ten kocyk jest cieńszy niż tamten.
• Ten kocyk jest cieńszy od tamtego.

Because niż does not force the same case that od does.

Here, tamten works like that one and stays in the nominative-style form:

• cieńszy niż tamten

But with od, Polish requires the genitive:

• cieńszy od tamtego

So:

• niż tamten = correct
• od tamtego = correct
• od tamten = incorrect

Niemowlę is grammatically neuter.

That is a grammar fact, even though a real baby can of course be male or female. In Polish, some nouns for young beings are neuter.

You can see this in agreement:

• małe niemowlę = a small baby
• to niemowlę = this baby

In the present tense, the verb form lubi does not show gender, so you do not notice it much here. But in the past tense you would:

• Niemowlę lubiło go najbardziej.

The -ło shows neuter agreement.

Because lubić takes a direct object in the accusative.

The baby likes the blanket, and kocyk is masculine, so the pronoun referring back to it is:

• go = him/it

Here it means it:

• niemowlę lubi go najbardziej = the baby likes it most

This is the normal short unstressed form. Compare:

• go = usual in a sentence like this
• jego = more emphatic, or used after some prepositions

So go is the natural choice here.

In Polish, when you talk about liking, loving, or similar verbs, najbardziej is very often the natural way to say most.

• lubi najbardziej = likes most

So:

• niemowlę lubi go najbardziej = the baby likes it the most

This is different from English, where most can be used more directly. In Polish:

• najbardziej often goes with verbs of preference or feeling
• najwięcej usually means the greatest quantity / the most in amount
• najlepiej usually means best

So these are different:

• lubi go najbardziej = likes it most
• ma najwięcej kocyków = has the most blankets
• zna to najlepiej = knows this best

No, Polish word order is fairly flexible. The given version is a neutral, natural one:

• Ten kocyk jest cieńszy niż tamten, ale niemowlę lubi go najbardziej.

You could rearrange parts for emphasis, for example:

• Ale niemowlę najbardziej lubi go.
• Ten kocyk niemowlę lubi najbardziej.

These versions are possible, but they shift emphasis or sound less neutral in everyday speech.

For a learner, the original sentence is a very good model for normal word order.

A rough guide:

• niż: like neezh
• niemowlę: starts with nye-, so roughly nye-MOV-leh
• cieńszy: begins with a soft sound, not a hard k; the sz sounds like English sh

A few sound notes:

• ń is a soft n, a bit like the ny sound in canyon
• sz sounds like English sh
• final ę is often only lightly nasal in normal speech

If you want a practical learner approach, aim for:

• niż → neezh
• niemowlę → nye-MOV-leh
• cieńszy → something close to chyen-shih

Not perfect, but good enough as a starting point.

Because ale means but, and in Polish it normally introduces a new clause, so it is preceded by a comma.

Here the sentence has two parts:

• Ten kocyk jest cieńszy niż tamten
• ale niemowlę lubi go najbardziej

So the comma is required:

• ..., ale ...

This is standard Polish punctuation.