Прокурор задал важный вопрос.

Because it is in the nominative case, which is the case normally used for the subject of a sentence.

• Прокурор = the prosecutor / prosecutor
• It is the person doing the action.

So in Прокурор задал важный вопрос, the prosecutor is the one who asked the question.

Задал is the masculine singular past tense of задать, which is a perfective verb.

Russian often uses:

• задавать = imperfective, focusing on process, repetition, or general fact
• задать = perfective, focusing on a completed action

So:

• Прокурор задал важный вопрос = The prosecutor asked an important question
  (one completed act)

If you said задавал, it would suggest something more like:

• he was asking
• he used to ask
• he asked repeatedly
• the action is being viewed as a process rather than a completed event

It actually is in the accusative case, but for masculine inanimate nouns, the accusative singular is often the same as the nominative singular.

So:

• nominative: важный вопрос
• accusative: важный вопрос

That is why the object looks unchanged.

This happens because:

• вопрос is masculine
• вопрос is inanimate

And the adjective matches it:

• важный also stays the same in masculine inanimate accusative singular

It is in the accusative case, because it is the direct object of the verb задал.

The prosecutor did the action, and the thing affected by the action is the important question.

So:

• Прокурор = nominative subject
• важный вопрос = accusative object

It comes from the infinitive задать.

Russian past tense is usually formed with -л plus endings for gender/number:

• masculine: задал
• feminine: задала
• neuter: задало
• plural: задали

In your sentence, the subject прокурор is masculine singular, so the verb is задал.

Russian has no articles.

So Прокурор can mean:

• the prosecutor
• a prosecutor

And важный вопрос can mean:

• an important question
• the important question

Which meaning is intended depends on context. In many simple example sentences, English adds the or a naturally, but Russian does not need them.

Yes. Russian word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Прокурор задал важный вопрос.

But you could also say:

• Важный вопрос задал прокурор.
• Задал прокурор важный вопрос.

These alternatives may sound more marked or emphasize different parts of the sentence, but the basic meaning remains similar because the cases still show who did what.

Because in Russian, adjectives usually come before the nouns they describe.

So:

• важный вопрос = an important question

That is the normal and expected order.
Russian can sometimes move adjectives after the noun for special stylistic effects, but that is not the standard pattern for ordinary speech.

Sometimes, but задать вопрос is the most standard and natural collocation for to ask a question.

Russian commonly says:

• задать вопрос = to ask a question

While спросить is also a verb meaning to ask, it is more often used with:

• a person: спросить прокурора
• information/about something: спросить о деле

So for ask a question, задал вопрос is the most idiomatic choice.

Usually, yes: прокурор generally means prosecutor or public prosecutor.

However, legal systems differ from country to country, so the exact job role may not match perfectly in every context. For language-learning purposes, prosecutor is the right translation.

The form вопрос here is singular.

If it were plural, you would see something different, for example:

• singular: важный вопрос
• plural: важные вопросы

So in this sentence, the prosecutor asked one important question.

The dictionary forms are:

• прокурор → прокурор
• задал → задать
• важный → важный
• вопрос → вопрос

Only задал is not already in dictionary form; it is a past-tense form of задать.