W czajniku gotuje się woda na herbatę.

Because w means in, and after w Polish normally uses the locative case when talking about location.

So:

• czajnik = kettle
• w czajniku = in the kettle

The ending changes from -ik to -iku because of the locative:

• ten czajnik
• w tym czajniku

This is very common in Polish:

• w domu = in the house
• w sklepie = in the shop
• w garnku = in the pot
• w czajniku = in the kettle

Here gotuje się means is boiling or is cooking.

The word się does not mean that the water is doing something to itself in a literal reflexive sense. In sentences like this, się is often used to make the verb sound natural for processes that happen on their own.

So:

• Woda gotuje się. = The water is boiling.
• literally, it is something like Water cooks/boils itself, but that is not how it should be understood in English.

This use is very common in Polish:

• Zupa się gotuje. = The soup is cooking.
• Mleko się gotuje. = The milk is boiling.

In practice, you should learn gotować się as a natural expression meaning to be boiling / to boil.

Yes, you can.

Polish word order is much more flexible than English word order. The sentence:

• W czajniku gotuje się woda na herbatę.

puts the location first, so it sounds a bit like:

• In the kettle, water for tea is boiling.

If you say:

• Woda gotuje się w czajniku.

that is also correct and very natural.

The difference is mostly about focus:

• W czajniku... focuses first on the place
• Woda... focuses first on the water

Polish often moves parts of the sentence around depending on what the speaker wants to emphasize.

Na herbatę means for tea.

It expresses purpose or intended use: this is water meant for making tea.

This is a very common Polish pattern:

• na obiad = for lunch
• na kolację = for dinner
• na kawę = for coffee
• na herbatę = for tea

So woda na herbatę is:

• water for tea
• tea water
• water intended to make tea

It does not mean the water is physically going onto the tea.

Because after na in this meaning of for / intended for, Polish uses the accusative case.

So:

• herbata = tea
• na herbatę = for tea

This is why the ending changes.

More examples:

• kawa → na kawę
• zupa → na zupę
• kolacja → na kolację

So na herbatę is grammatically the same kind of structure.

Yes. Gotować can mean both to cook and to boil, depending on context.

Examples:

• Gotuję obiad. = I’m cooking dinner.
• Woda się gotuje. = The water is boiling.

So with food, it often means cook.
With liquids like water, it often means boil.

That is why gotuje się woda is perfectly normal Polish.

Not exactly, though they are related.

• gotować się = to be boiling / to cook
• wrzeć = to boil, to be at a full boil

Wrzeć can sound a bit more precise or stronger, while gotować się is very common in everyday speech.

For example:

• Woda się gotuje. = The water is boiling.
• Woda wrze. = The water is boiling / at a boil.

In ordinary conversation, learners will hear woda się gotuje very often.

It is present tense of the imperfective verb gotować się.

In Polish, the present tense of an imperfective verb can describe:

• something happening right now
• a general repeated action
• an ongoing process

Here it describes an action happening now:

• gotuje się = is boiling

So even though it is morphologically present tense, in English it is often translated with the present continuous:

• The water is boiling.

Not if you mean the sentence given here.

• woda na herbatę = water for tea
• herbata = tea itself

So W czajniku gotuje się woda na herbatę means the kettle contains water that will be used to make tea.

If you said:

• W czajniku gotuje się herbata

that would suggest that tea itself is boiling in the kettle, which is not the usual idea.

Yes, it is natural.

It sounds like a normal descriptive sentence, especially if the speaker wants to begin with the location:

• W czajniku gotuje się woda na herbatę.

A very common alternative would be:

• Woda na herbatę gotuje się w czajniku.
• Woda gotuje się w czajniku.

All of these are grammatical. The original sentence is fine; it just gives a little prominence to where the action is happening.

Usually yes, but only within normal Polish word-order rules.

In this sentence, the natural version is:

• W czajniku gotuje się woda na herbatę.

You could also have:

• Woda się gotuje w czajniku.

But you cannot place się absolutely anywhere. It tends to stay close to the verb and follows common clitic-position rules.

For a learner, the safest approach is to memorize natural patterns such as:

• woda się gotuje
• zupa się gotuje
• obiad się gotuje

That will sound much better than trying to move się freely.