Learn Hardcore Ukrainian: Чайник свистить, коли вода закипає. - The kettle whistles when the water boils.

Ukrainian does not have articles—there are no direct equivalents of the or a. Definiteness or indefiniteness is inferred from context or by using demonstratives like цей (“this”) or той (“that”) when necessary.

Why is чайник in the nominative case here?

чайник is the subject of the main clause (чайник свистить, “the kettle whistles”), and subjects take the nominative case in Ukrainian.

Why is вода also in the nominative case, instead of another case?

Here вода is the subject of the subordinate time clause introduced by коли (“when”), so it remains in the nominative. If вода were an object, you would use the accusative.

What does the conjunction коли mean, and could I use як or як тільки instead?

коли means “when” for general or repeated time clauses. як can colloquially mean “when,” and як тільки means “as soon as” (emphasizing immediacy).
Example: Як тільки вода закипає, чайник свистить (“As soon as the water boils, the kettle whistles”).

Why is свистить in the imperfective aspect instead of a perfective form?

Imperfective verbs like свистити describe ongoing, habitual, or repeated actions. Since this is a general rule (“whenever the water boils, the kettle whistles”), we use свистить. A perfective form (e.g., просвистити) would imply a single, completed whistle event.

Why is закипає in imperfective, and what about закипить or кипить?

закипає is the 3rd-person singular present of the imperfective verb закипати (“to start boiling” or “to boil up”), highlighting the process itself.

закипить (perfective) would emphasize the moment the water reaches boiling point (completion).
кипить (imperfective of кипіти) means “is boiling” in the sense of vigorous bubbling.
Here закипає best conveys “when the water begins to boil.”

Why is there a comma before коли and not after?

In Ukrainian, when the main clause comes first and is followed by a subordinate clause (introduced by коли), you place a comma immediately before the conjunction. If you reverse them, put the comma after the subordinate clause:
Чайник свистить, коли вода закипає.
Коли вода закипає, чайник свистить.

Can I change the word order to Коли закипає вода, чайник свистить, and does it change the meaning?

Yes, Ukrainian has relatively free word order. Коли закипає вода, чайник свистить is grammatically correct and simply shifts emphasis to закипає вода. The overall meaning remains the same.

Why don’t we specify в ньому (“in it”) to say “the water in the kettle”?

It’s clear from context that the water is in the kettle, so в ньому is redundant. If you want to be explicit, you could say коли вода в ньому закипає, but it isn’t necessary here.

Where does the stress fall in свистить and закипає?

In свистить, the stress is on the second syllable: свиСти́ть.
In закипає, the stress is on the third syllable: закиПА́є.