Spokojny oddech jest ważny podczas zejścia.

Because they both agree with oddech, which is a masculine singular noun.

• spokojny oddech: the adjective directly describes oddech
• oddech jest ważny: the predicate adjective ważny also matches the subject

If the noun changed, the adjective forms would change too:

• spokojna cisza jest ważna
• spokojne oddychanie jest ważne

So this is standard Polish adjective agreement.

Because podczas requires the genitive case, and the genitive singular of zejście is zejścia.

So the pattern is:

• nominative: zejście
• genitive: zejścia

That is why you get podczas zejścia, not podczas zejście.

In normal standard Polish, jest is the natural form here and should be kept.

Polish does not usually drop the present-tense form of to be in ordinary sentences like this.
Without jest, the sentence would sound incomplete, very elliptical, or like a note/headline rather than a normal full sentence.

So:

• natural: Spokojny oddech jest ważny podczas zejścia.
• not normal as a full sentence: Spokojny oddech ważny podczas zejścia.

Yes. Polish word order is fairly flexible because the endings show the grammatical relationships.

For example, all of these are possible:

• Spokojny oddech jest ważny podczas zejścia.
• Podczas zejścia spokojny oddech jest ważny.
• Ważny jest spokojny oddech podczas zejścia.

The basic meaning stays the same, but the emphasis changes:

• starting with podczas zejścia puts the situation first
• starting with ważny emphasizes what is important

The original version is neutral and natural.

Not necessarily. In Polish, oddech can refer not only to a single breath, but also to someone’s breathing or breathing pattern in a more general sense.

So spokojny oddech can mean something like calm, steady breathing, not just one isolated inhale or exhale.

That is why this sentence sounds natural in instruction, sports, medical, or wellness contexts.

Both are possible, but they are not exactly the same.

• oddech = breath, breathing, breathing pattern
• oddychanie = breathing as a process or activity

Spokojny oddech sounds concise and idiomatic when talking about keeping the breath calm and steady.
Spokojne oddychanie is also correct, but it feels a little more like talking about the act or process of breathing.

So the original wording is very natural.

Both can work, but they emphasize slightly different things.

• zejście often refers to a specific descent as a whole, especially in contexts like hiking, climbing, or going down from somewhere
• schodzenie emphasizes the ongoing process of going down

So:

• podczas zejścia = during the descent
• podczas schodzenia = while going down

In many contexts the difference is small, but zejście often sounds more like a bounded event, while schodzenie sounds more process-focused.

Because Polish has no articles.

English needs words like a and the, but Polish normally does not use anything equivalent.
Whether something is definite or indefinite is understood from context.

So spokojny oddech can mean:

• a calm breath
• calm breathing
• the calm breath

depending on the situation.

No. Podczas is very natural, but you could also say w czasie.

For example:

• podczas zejścia
• w czasie zejścia

Both mean essentially the same thing here.
Podczas is compact and common in both spoken and written Polish, while w czasie can sound a little more explicit or slightly less compact.

A rough guide:

• oddech ≈ OD-dehkh
• zejścia ≈ ZEYSH-cha, but with very soft Polish sounds in the second part

A few details:

• ch in oddech is not English ch as in chair; it is the rough sound heard in German Bach or Scottish loch
• ś and ć are soft consonants, so ścia is softer than an English shcha
• Polish stress is usually on the next-to-last syllable, so the stress falls naturally near the beginning in both of these words

If pronunciation is your goal, zejścia is the trickier word because of the consonant cluster.