Learn Hardcore Ukrainian: Я прошу пояснення, коли тема складна. - I ask for an explanation when the topic is difficult.

Because коли introduces a subordinate time clause. In Ukrainian, subordinate clauses are separated by a comma. You can also flip the order: Коли тема складна, я прошу пояснення. The comma stays either way.

Why is there no word for “is” in тема складна?

In the present tense, Ukrainian usually omits the verb бути (to be). So Тема складна literally reads “The topic difficult,” which means “The topic is difficult.”

Past/future use the copula and the adjective in the instrumental: Тема була складною. / Тема буде складною.
You can include the present є in formal style: Тема є складною, but everyday speech normally drops it.

Which case is пояснення in, and why does it look the same in several cases?

Nouns in -ння (like пояснення) have the same form for several singular cases. Here, after просити, it can be understood as either accusative singular (asking for an explanation as a whole) or genitive singular (partitive, “some explanation”). The form is identical: пояснення. Context does the work.

Why is пояснення singular? Could I use a plural?

Singular is normal when you mean “an explanation” generally. You have options:

Я прошу пояснення. (singular: “an explanation”)
Я прошу пояснень. (genitive plural: “explanations/clarification(s)”) Both are correct; choose based on whether you want one vs multiple/unspecified explanations.

Do I need the preposition про in просити (про) пояснення?

Both are acceptable:

просити пояснення (most common, concise, neutral)
просити про пояснення (also correct; a bit more explicit/formal) You’ll see both in real usage.

What’s the difference between просити and питати/запитувати?

просити = “to ask for” something: Я прошу пояснення (I ask for an explanation).
питати/запитувати = “to ask” a question: Я питаю вчителя (I ask the teacher [a question]). Using питати here would change the meaning.

Could I say Я прошу пояснити instead of using the noun пояснення?

Yes. That shifts the request from “an explanation” (a thing) to “to explain” (an action):

Я прошу пояснити, коли тема складна. You can also add who should benefit from it:
Я прошу, щоб мені пояснили, коли тема складна.

What about aspect: прошу vs попрошу?

прошу (imperfective) expresses a general/habitual action: “I ask (whenever…).”
попрошу (perfective) is one-off/future: “I will ask (on that occasion).” Your sentence is habitual, so прошу fits.

Does коли mean “when” or “whenever” here?

With present tense in both clauses, коли conveys a habitual meaning: “whenever the topic is difficult.” For a conditional nuance, якщо (“if”) is also possible: Я прошу пояснення, якщо тема складна.

Is Коли тема складна, я прошу пояснення equally correct?

Yes. Ukrainian word order is flexible. Starting with the time clause adds emphasis to the condition/time frame. Both orders are natural.

Why does складна end in -а?

Agreement. тема is feminine singular, so the predicate adjective is feminine singular складна. For other nouns:

питання (neuter): питання складне
матеріал (masc.): матеріал складний

Could I use важкий instead of складний?

Often, yes, but nuance differs:

складний = complex/complicated in structure.
важкий = hard/difficult (heavy figuratively or literally). For abstract topics, both occur, but складний highlights complexity.

Are articles (“a/the”) missing on purpose?

Ukrainian has no articles. тема can mean “a topic” or “the topic” depending on context. If you need to be specific, use demonstratives: ця тема (this topic).

Should я be capitalized like English “I”?

No. я is lowercase in Ukrainian. It’s capitalized only because it begins the sentence.

How do you pronounce the sentence and where is the stress?

Я прошу́ поясне́ння, коли́ те́ма складна́. Stresses: я proshú poyasnénnia, kolý téma skladná. Notes:
прошу́ stresses the last syllable.
поясне́ння stresses -не́-.
коли́ stresses the last syllable.
складна́ stresses the last syllable.

How do I say who I’m asking?

Use в/у + Genitive for the person:

Я прошу пояснення в учителя. (I ask the teacher for an explanation.)
Я прошу пояснення у колеги. (I ask a colleague for an explanation.)

Would коли тема стає складною change the meaning?

Slightly. коли тема складна states a general condition/state. коли тема стає складною focuses on the moment of becoming difficult (the transition), so it’s about when the difficulty arises.