Ten, kto zauważy dym na klatce schodowej, powinien od razu wezwać pomoc.

Ten, kto ... is a common Polish structure meaning something like the one who ... or whoever ....

So:

• ten = that one / the one
• kto = who

Together, Ten, kto zauważy dym... means The person who notices smoke...

You can sometimes say just Kto zauważy dym, powinien..., and that would also be understandable. But ten, kto sounds a bit more formal and explicit.

Zauważy is from the perfective verb zauważyć.

In Polish, perfective verbs often use present-looking forms to express a future completed action:

• zauważa = notices / is noticing / notices repeatedly
  from imperfective zauważać
• zauważy = will notice / notices at the moment it happens
  from perfective zauważyć

In this sentence, Polish uses the perfective because it refers to the moment when someone becomes aware of the smoke: whoever notices smoke.

This is very natural in rules, instructions, and warnings.

This is one of the biggest differences between English and Polish.

With perfective verbs, Polish does not normally form a true present tense. The present-style endings refer to the future.

So:

• zauważa = is noticing / notices
• zauważy = will notice

But in English, after words like who, if, when, whoever, we often use present forms for future meaning too:

• Whoever notices smoke should call for help

So although zauważy is grammatically future in Polish, the English translation may still use notices, depending on context.

Yes, powinien is grammatically masculine singular. But in Polish, the masculine singular form is often used in a generic sense, especially in formal instructions, regulations, or general statements.

So here powinien means:

• he should, literally
• but in practice: the person should, one should, whoever it is should

This is very common in Polish.

Other more impersonal ways to say something similar would be:

• należy od razu wezwać pomoc
• powinno się od razu wezwać pomoc

Those sound more impersonal and official.

Dym is the direct object of zauważy, so it is in the accusative case.

However, for many masculine inanimate nouns in Polish, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: dym
• accusative: dym

That is why there is no visible change.

This is partly a matter of idiomatic usage.

In Polish, na klatce schodowej is the normal, natural way to say in the stairwell / on the staircase landing / in the common stair area.

Even though English would often use in, Polish commonly uses na for certain areas or surfaces understood as shared spaces.

So:

• na klatce schodowej = the usual phrasing

Grammatically, na here takes the locative case, which is why you get:

• klatce
• schodowej

Because after na in this meaning, Polish uses the locative case.

Base form:

• klatka schodowa = staircase / stairwell

Locative singular:

• na klatce schodowej

Both the noun and the adjective change because Polish adjectives must agree with the nouns they describe.

So:

• klatka → klatce
• schodowa → schodowej

This is standard noun-adjective agreement in case, number, and gender.

Od razu means:

• immediately
• right away
• at once

In this sentence it emphasizes that the person should not wait.

Very common Polish expression:

• Zrób to od razu. = Do it right away.

Because wezwać takes a direct object in the accusative case.

The noun pomoc is feminine, and in this case the accusative singular looks the same as the nominative:

• nominative: pomoc
• accusative: pomoc

So:

• wezwać pomoc = to call for help / summon help

The form pomocy is a different case, not the one required here.

You may also hear other expressions such as:

• wezwać straż pożarną = call the fire brigade
• zadzwonić po pomoc = call for help

But with wezwać, pomoc is the correct form here.

Because that part is a clause inserted after ten and it explains which person is being talked about.

Structure:

• Ten
• kto zauważy dym na klatce schodowej
• powinien od razu wezwać pomoc

So the commas mark the embedded clause introduced by kto.

You can think of it as similar to:

• The one who notices smoke in the stairwell should immediately call for help.

Polish punctuation regularly separates this kind of inserted clause with commas.

Yes. The given sentence is correct and natural, especially in a formal warning or instruction. But Polish offers a few alternatives with slightly different style:

• Kto zauważy dym na klatce schodowej, powinien od razu wezwać pomoc.
  Slightly shorter, still natural.

• W razie zauważenia dymu na klatce schodowej należy od razu wezwać pomoc.
  More formal, official, instruction-like.

• Jeśli ktoś zauważy dym na klatce schodowej, powinien od razu wezwać pomoc.
  A little more conversational.

So the original sentence is perfectly good, but it has a somewhat formal, written feel.