Ta gaśnica jest ciężka, ale każdy dorosły powinien wiedzieć, jak nią gasić mały pożar.

Because gaśnica is the subject of the sentence: Ta gaśnica jest ciężka.

Subjects normally appear in the nominative case, so you get:

• ta gaśnica = nominative

By contrast, tę gaśnicę is accusative, which you would use when the extinguisher is the direct object:

• Widzę tę gaśnicę. = I see this extinguisher.

So here, since the sentence is saying what the extinguisher is like, nominative is required.

Because adjectives in Polish must agree with the noun they describe in gender, number, and case.

Gaśnica is:

• feminine
• singular
• nominative

So the adjective must match:

• ciężka = feminine singular nominative

Compare:

• ciężki stół = heavy table
• ciężka gaśnica = heavy extinguisher
• ciężkie krzesło = heavy chair

Yes. In Polish, an adjective can sometimes be used like a noun. This is often called a substantivized adjective.

So:

• dorosły literally means adult
• but it can also mean an adult person

You could think of it as short for something like:

• dorosły człowiek = an adult person

But in normal Polish, just dorosły is completely natural.

Similarly:

• młody can mean a young man / young person in some contexts
• chory can mean a sick person / patient

Two things are happening here.

1. Każdy means each / every, so it takes a singular noun phrase and a singular verb form:

• Każdy dorosły powinien... = Every adult should...

If you wanted all adults, you would normally use the plural:

• Wszyscy dorośli powinni...

2. The masculine form is often used in Polish as a generic form when the person’s gender is not specified.

So:

• każdy dorosły powinien wiedzieć = every adult should know

If you were specifically talking about a woman, you would say:

• każda dorosła powinna wiedzieć

Because Polish commonly expresses know how to do something as:

• wiedzieć, jak + infinitive

So:

• wiedzieć, jak nią gasić mały pożar
  = to know how to extinguish a small fire with it

This is a very common pattern:

• Wiem, jak to zrobić. = I know how to do it.
• Nie wie, jak gotować. = He/She doesn’t know how to cook.

So jak here means how, not like.

Because nią is the form used here for the idea with it / using it.

The noun gaśnica is feminine, and the pronoun referring to it is based on ona. Different cases give different forms:

• ją = accusative
• nią = instrumental

In this sentence, the extinguisher is the tool used to do the action, so Polish uses the instrumental:

• gasić gaśnicą = to extinguish with an extinguisher
• gasić nią = to extinguish with it

So ją would not fit here, because that would make the pronoun more like a direct object.

Because Polish often marks the instrument or means of an action with the instrumental case.

This is very common with tools:

• pisać długopisem = write with a pen
• kroić nożem = cut with a knife
• otworzyć kluczem = open with a key
• gasić gaśnicą = extinguish with a fire extinguisher

When you replace the noun with a pronoun, the instrumental stays:

• gaśnicą → nią

So the structure is basically:

• jak nią gasić mały pożar = how to put out a small fire with it

This is about aspect.

• gasić = imperfective
• ugasić = perfective

In this sentence, the speaker is talking about a general skill or procedure: what an adult should know how to do. For that, the imperfective gasić is very natural.

So:

• wiedzieć, jak gasić = know how to extinguish / know how to go about extinguishing

If you said:

• wiedzieć, jak ugasić mały pożar

that would sound more focused on successfully finishing one whole action: how to put out a small fire completely.

Both can make sense, but gasić fits the idea of general know-how especially well.

Also, with pożar, ugasić is usually more natural than zgasić.

It is accusative singular, because it is the direct object of gasić.

You extinguish what?

• mały pożar

The reason it looks the same as the dictionary form is that pożar is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: mały pożar
• accusative: mały pożar

Compare that with a masculine animate noun:

• nominative: mały pies
• accusative: małego psa

That contrast helps show what is happening.

They both relate to fire, but they are not used in exactly the same way.

• pożar = a fire as a dangerous event, blaze, fire emergency
• ogień = fire in general, flame, burning fire

So in safety language, pożar is the natural choice:

• mały pożar = a small fire
• pożar domu = a house fire
• pożar lasu = a forest fire

Whereas ogień is broader:

• ogień w kominku = fire in the fireplace
• bawić się ogniem = play with fire

A fire extinguisher is something you use in response to a pożar, so this word fits the context very well.

Polish word order is fairly flexible, so yes, you can move things around more than in English. But the version in the sentence sounds very natural:

• jak nią gasić mały pożar

Putting nią early works well because short pronoun-like elements often appear relatively early in the phrase.

Other orders may be possible, but they can sound:

• less neutral
• more emphatic
• or slightly less natural in ordinary speech

So a learner should treat jak nią gasić mały pożar as a good standard model.