Kolorowy szalik na jego szyi sprawia, że wygląda na szczęśliwego.

The sentence breaks down like this:

• Kolorowy szalik na jego szyi = The colorful scarf on his neck
• sprawia, że = makes it so that / causes that
• wygląda na szczęśliwego = he looks happy / he seems happy

So the whole sentence means something like:

The colorful scarf on his neck makes him look happy.

Grammatically, the main subject is szalik (scarf), and the second part describes the effect that scarf has.

Because szalik is the subject of the sentence, so it is in the nominative case.

• szalik = nominative singular, scarf
• szalika = usually genitive singular or accusative singular for some uses, not correct here as the subject

In Kolorowy szalik na jego szyi sprawia..., the scarf is the thing that does the action of sprawia (causes / makes), so nominative is required.

Kolorowy is an adjective meaning colorful, and it has to agree with szalik in:

• gender: masculine
• number: singular
• case: nominative

Since szalik is masculine singular nominative, the adjective also takes the masculine singular nominative form:

• kolorowy szalik

This is standard adjective-noun agreement in Polish.

Here, szyi is in the locative singular.

The noun is:

• szyja = neck

After the preposition na when it means on or at a location, Polish often uses the locative:

• na szyi = on the neck / around the neck

So:

• jego szyja = his neck (dictionary/base form would be szyja)
• na jego szyi = on his neck

Even though English would often say around his neck, Polish very naturally uses na szyi.

Jego here means his.

In na jego szyi, it modifies szyi:

• jego szyi = his neck

This possessive form does not change here, even though the noun changes case. So you get:

• jego szyja = his neck
• na jego szyi = on his neck

Polish personal/possessive pronouns can be tricky, but jego is the normal word for his in this kind of phrase.

Sprawia, że is a very common Polish structure meaning:

• makes it so that...
• causes that...
• more naturally in English: makes ..., causes ... to ...

Examples:

• To sprawia, że się uśmiecham. = That makes me smile.
• Muzyka sprawia, że czuję się lepiej. = Music makes me feel better.

In your sentence:

• Kolorowy szalik ... sprawia, że wygląda na szczęśliwego.

Literally: The colorful scarf ... causes that he looks happy.

Natural English: The colorful scarf on his neck makes him look happy.

Wygląda is the 3rd person singular form of wyglądać (to look / appear / seem).

It means:

• he/she/it looks

In this sentence, the understood person is he. So:

• wygląda = he looks / he seems

Even though he is not explicitly repeated, it is understood from the context. Polish often leaves out subject pronouns when the meaning is clear.

So the sentence does not mean that the scarf itself looks happy. It means the scarf makes him look happy.

This is one of the most important grammar points in the sentence.

Polish often uses:

• wyglądać na + accusative

to mean:

• to look like
• to seem
• to appear to be

So:

• wygląda na zmęczonego = he looks tired
• wygląda na chorego = he looks ill
• wygląda na szczęśliwego = he looks happy

That is why the adjective appears after na and in the accusative form.

For a masculine person, the accusative form often looks like the genitive:

• szczęśliwy → szczęśliwego

So na szczęśliwego is correct here.

You can think of it that way, yes.

Historically and grammatically, expressions like wygląda na szczęśliwego can be understood as something like:

• wygląda na szczęśliwego człowieka = he looks like a happy person

Very often, the noun is simply omitted because it is obvious.

So learners often find na szczęśliwego strange at first, but it is a normal Polish pattern with wyglądać na.

That would mean something different.

• wygląda na szczęśliwego = he looks happy / he seems happy
• wygląda szczęśliwie would sound like he looks in a happy way, which is not the normal idiomatic choice here

In Polish, when talking about someone’s apparent state or condition, wyglądać na + accusative is very common.

So in this sentence, na szczęśliwego is the natural option.

Yes, Polish word order is fairly flexible, though some orders sound more natural than others.

The original sentence is natural:

• Kolorowy szalik na jego szyi sprawia, że wygląda na szczęśliwego.

You could also move things for emphasis, for example:

• Na jego szyi kolorowy szalik sprawia, że wygląda na szczęśliwego.

But that sounds more marked or stylistic.

For a learner, the original version is the safest and most natural.

English says:

• The colorful scarf on his neck makes him look happy.

Polish instead uses a clause with że:

• sprawia, że wygląda na szczęśliwego

This literally means:

• makes it so that he looks happy

Polish does not need to repeat him as a separate object in the same way English does. The meaning is carried by the whole clause after że.

So this is a normal difference between English and Polish sentence structure.

Yes, it is very natural.

Polish often says:

• mieć coś na szyi = to have something on/around one’s neck

So a scarf, necklace, or tie can all be described with na szyi.

Even if English prefers around his neck, Polish na jego szyi is completely normal and idiomatic.