Kiedy grzejnik nie działa, mama zakłada jeszcze jeden sweter.

Here kiedy can be understood as when, but in this kind of present-tense sentence it often has the sense of whenever.

So:

Kiedy grzejnik nie działa, mama zakłada jeszcze jeden sweter.

can mean:

When / Whenever the radiator doesn’t work, Mom puts on another sweater.

The Polish form itself is normal for both ideas; context tells you whether it is about one situation or a repeated habit.

Kiedy introduces a time relationship: when.

Jeśli introduces a condition: if.

In this sentence, Polish presents the situation as a time-based pattern: when the radiator isn’t working, this is what Mom does. That is why kiedy sounds natural.

You could use jeśli in some contexts, but it would make the sentence feel more conditional:

Jeśli grzejnik nie działa, mama zakłada jeszcze jeden sweter.
= If the radiator isn’t working, Mom puts on another sweater.

Both are possible, but kiedy fits the original sentence well.

Because Kiedy grzejnik nie działa is a subordinate clause introduced by kiedy.

In Polish, subordinate clauses are normally separated from the main clause by a comma:

Kiedy grzejnik nie działa, mama zakłada jeszcze jeden sweter.

This is true whether the subordinate clause comes first or second.

Because they are the subjects of their verbs, so they are in the nominative case.

• grzejnik is the subject of nie działa
• mama is the subject of zakłada

So both appear in their dictionary form:

• grzejnik = radiator
• mama = mom

Because sweter is the direct object of zakłada, so it is in the accusative case.

However, sweter is a masculine inanimate noun, and in the singular its accusative form is the same as its nominative form. So:

• nominative: sweter
• accusative: sweter

That is why the form does not change.

The same kind of thing happens in many Polish masculine inanimate nouns.

Działać means to work, to function, or to operate.

So grzejnik nie działa means:

• the radiator doesn’t work
• the radiator isn’t working
• the radiator is not functioning

This is the most natural verb here. For a device or appliance, działać is very common.

Because działać is the normal verb for something functioning properly.

• urządzenie działa = a device works/functions
• grzejnik działa = a radiator works

Pracować usually means to work in the sense of doing work, being employed, or sometimes operating actively, but for a radiator grzejnik nie działa is much more natural than grzejnik nie pracuje.

Here zakładać means to put on an item of clothing.

So:

• zakłada sweter = puts on a sweater

This verb is very commonly used for clothes, shoes, glasses, hats, and similar things.

In other contexts, zakładać can have other meanings too, such as to found, to assume, or to establish, so the clothing context is important here.

This is about aspect.

• zakładać = imperfective
• założyć = perfective

In the sentence, zakłada is present tense of the imperfective verb zakładać. That fits well because the sentence describes a repeated or habitual action:

Whenever the radiator doesn’t work, Mom puts on another sweater.

A perfective present form like założy usually has future meaning, not ordinary present meaning:

• Mama założy jeszcze jeden sweter.
  = Mom will put on one more sweater.

So zakłada is the correct choice for a general habit.

It means one more sweater or another sweater.

The key word is jeszcze, which here adds the idea of an additional one.

So:

• jeden sweter = one sweater
• jeszcze jeden sweter = one more sweater / another sweater

This suggests that Mom is already wearing something warm, and then adds an extra sweater.

Because Polish often leaves subject pronouns unstated when they are not needed.

In this sentence, the subject is already clearly named as mama, so adding ona would usually be unnecessary.

Polish does this much more often than English does. The verb form also helps show who is doing the action.

So the natural sentence is simply:

mama zakłada

not usually:

ona mama zakłada
and not normally mama ona zakłada

If ona were added, it would sound emphatic or contrastive in most contexts.

Yes. Polish word order is fairly flexible.

The original order is neutral and natural:

Kiedy grzejnik nie działa, mama zakłada jeszcze jeden sweter.

You could also say:

Mama zakłada jeszcze jeden sweter, kiedy grzejnik nie działa.

That still means basically the same thing. The difference is mostly in emphasis and information flow.

Putting the kiedy clause first makes the time/situation the starting point of the sentence, which sounds very natural here.