Sobotni poranek jest spokojny.

Because sobotni is an adjective, while sobota is a noun meaning Saturday.

In this sentence, sobotni means Saturday / Saturday-related and describes poranek (morning). So Polish works like English Saturday morning, except Polish usually makes Saturday into an adjective:

• sobota = Saturday
• sobotni = Saturday, Saturday-related

So:

• sobotni poranek = Saturday morning

You cannot normally say sobota poranek in standard Polish, because that would be like putting two nouns together where Polish expects an adjective + noun structure.

It ends in -i because it has to agree with the noun poranek.

In Polish, adjectives change their endings to match the noun in:

• gender
• number
• case

Poranek is:

• masculine
• singular
• nominative

So the adjective also takes the masculine singular nominative form:

• sobotni poranek

This is just the correct dictionary-style form for this masculine noun in this context.

For the same reason: adjective agreement.

Spokojny describes poranek, so it must match poranek in gender, number, and case.

Since poranek is masculine singular nominative, the adjective appears as:

• spokojny

Compare how the adjective changes with different nouns:

• spokojny poranek = calm morning
• spokojna noc = calm night
• spokojne dziecko = calm child
• spokojni ludzie = calm people

So the -y ending here is part of the masculine singular nominative form.

It is in the nominative case.

That is because sobotni poranek is the subject of the sentence:

• Sobotni poranek = the thing being described
• jest spokojny = is calm

In simple Polish sentences with X is Y, the subject is normally in the nominative.

Also, spokojny is nominative too, because it agrees here with the subject in this type of sentence.

Jest is the 3rd person singular form of być (to be), so it means is.

• Sobotni poranek jest spokojny. = Saturday morning is calm.

In the present tense, Polish often omits forms of to be in some contexts? Actually, with normal predicate adjective sentences like this, jest is usually kept in standard Polish.

So:

• Sobotni poranek jest spokojny. = normal, standard
• Sobotni poranek spokojny. = sounds incomplete or poetic/headline-like, not the normal full sentence

So for a learner, it is safest to use jest here.

No, Polish word order is fairly flexible, because the endings show the grammar.

The most neutral order here is:

• Sobotni poranek jest spokojny.

But other orders are possible for emphasis or style, for example:

• Poranek jest spokojny. = The morning is calm.
• Spokojny jest sobotni poranek. = It is the Saturday morning that is calm.
• Jest spokojny sobotni poranek. = more marked/literary

Even though these are possible, the original sentence is the most natural and neutral for everyday use.

Because poranek is a noun, while rano is usually an adverb.

• poranek = morning
• rano = in the morning / early in the day

In this sentence, morning is the subject, so Polish needs a noun:

• Sobotni poranek jest spokojny.

You would use rano differently, for example:

• W sobotę rano jest spokojnie. = On Saturday morning, it is calm.

So poranek names the thing; rano describes when something happens.

Yes. Ranek and poranek are very close in meaning, and in many contexts both can mean morning.

• sobotni ranek
• sobotni poranek

Both are acceptable.
Often poranek can feel a little more descriptive or literary, while ranek is also very common and natural. In everyday speech, either may work depending on style and habit.

So the sentence could also be:

• Sobotni ranek jest spokojny.

Here spokojny means something like:

• calm
• peaceful
• quiet

For a morning, it suggests there is no rush, noise, stress, or disturbance.

It does not necessarily mean silent in a strict sense. It is broader than that. A spokojny poranek could still have birds, conversation, or soft sounds; it just feels peaceful.

A rough English-friendly guide:

• sobotni ≈ so-BOT-nee
• poranek ≈ po-RA-nek
• jest ≈ yest
• spokojny ≈ spo-KOY-nih

A few useful pronunciation notes:

• Polish j sounds like English y in yes
• Polish cz is not here, but sp and tn clusters may feel tighter than in English
• ó is pronounced like u, but there is no ó in this sentence
• stress in Polish is usually on the second-to-last syllable:
  • so-BOT-ni
  • po-RA-nek
  • spo-KOJ-ny

No. In Polish, days of the week are not capitalized unless they begin a sentence.

So normally you write:

• sobota
• sobotni

not:

• Sobota
• Sobotni

In the sentence Sobotni poranek jest spokojny, the first word is capitalized only because it starts the sentence.

Not literally in the possessive English sense, but that is close to the idea.

Polish usually expresses this with an adjective:

• sobotni poranek

This is best understood as:

• Saturday morning
• the morning on Saturday
• a Saturday morning

So it is not a possessive form like English Saturday’s, but functionally it gives a very similar meaning in this phrase.

It can describe many nouns connected with Saturday.

For example:

• sobotni wieczór = Saturday evening
• sobotni mecz = Saturday match
• sobotnie spotkanie = Saturday meeting
• sobotnia kawa = Saturday coffee

The ending changes depending on the noun:

• sobotni for some masculine nouns
• sobotnia for feminine nouns
• sobotnie for neuter nouns

So sobotni is just one form of the adjective.