Woda w czajniku już się gotuje.

Because after w meaning in, Polish normally uses the locative case when you are talking about location.

• dictionary form: czajnik
• after w for location: w czajniku

So w czajniku means in the kettle. This is a very common pattern:

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

Here się is part of the verb expression gotować się.

With woda, gotować się means to boil / to be boiling. It does not mean the water is somehow doing something to itself in the literal English sense.

So:

• Woda się gotuje = The water is boiling.

This is one of many Polish verbs where się is just a normal part of how the verb is used in this meaning.

Because Polish does not form the present continuous the way English does.

In English, you say:

• The water is boiling

In Polish, the present-tense verb form gotuje already does that job. Depending on context, gotuje się can correspond to:

• boils
• is boiling

So Woda w czajniku już się gotuje naturally means The water in the kettle is already boiling or The water in the kettle is boiling already.

It depends on the subject and context.

With woda, the natural meaning is to boil:

• Woda się gotuje = The water is boiling.

With food, related forms can mean to cook or to be cooking. So the core idea is connected with cooking/boiling, but in this sentence, because the subject is woda, the meaning is clearly boil.

Już usually means already.

In this sentence, it shows that the boiling has begun or has reached the expected stage:

• Woda w czajniku się gotuje = The water in the kettle is boiling.
• Woda w czajniku już się gotuje = The water in the kettle is already boiling.

In natural English, już can sometimes sound like already, now, or just add a sense of it has started by this point.

Yes. Polish word order is more flexible than English word order, because the endings carry a lot of grammatical information.

The version you have:

• Woda w czajniku już się gotuje.

is natural and neutral.

Other orders are possible, for example:

• Już się gotuje woda w czajniku.
• Woda już się gotuje w czajniku.

These can sound a little different in emphasis. Polish often moves words around to highlight what is new, important, or contrastive.

One useful note: się is flexible, but it usually does not stand at the very beginning of a sentence.

Because this sentence describes an ongoing process or current state, not a completed result.

Gotować się is imperfective, so it is the normal choice for:

• something happening now
• an action in progress
• a repeated action

So:

• Woda się gotuje = The water is boiling.

A perfective verb like zagotować się focuses on reaching the endpoint, more like come to a boil. In the present tense, perfective verbs usually have a future meaning in Polish:

• Woda się zagotuje = The water will come to a boil.

That is why gotuje się is the right choice here.

Because Polish has no articles. There is no direct equivalent of English a / an / the.

So woda can mean:

• water
• the water

and w czajniku can mean:

• in a kettle
• in the kettle

Context tells you which one is meant. In this sentence, English naturally uses the:

• The water in the kettle is already boiling.