На семинаре можно задать сколько угодно вопросов, если тема непонятна.

Russian uses на with some events, activities, and organized gatherings, including семинар.

So:

• на семинаре = at the seminar / in the seminar session
• в семинаре would usually sound wrong here

This is similar to:

• на уроке = in class
• на лекции = at the lecture
• на встрече = at the meeting

Also, семинаре is in the prepositional case because it follows на in a location meaning.

• nominative: семинар
• prepositional: на семинаре

Можно means something like:

• it is possible
• one may
• you can / it’s allowed to

Russian often uses impersonal constructions, where there is no explicit subject like you or one.

So:

• На семинаре можно задать... = At the seminar, you can ask...
• literally: At the seminar, it is possible to ask...

This is very common in Russian:

• Здесь можно курить. = You can smoke here.
• Нельзя входить. = You must not enter / Entering is not allowed.

Because можно is typically followed by an infinitive.

Structure:

• можно + infinitive

So:

• можно задать = can ask / it is possible to ask

Other examples:

• можно спросить = may I ask / one can ask
• можно прийти = one can come

The infinitive is used because the sentence is not conjugating a personal verb like ты задаёшь or вы задаёте. Instead, it says that the action is generally possible.

Задать вопрос is the standard Russian expression for to ask a question.

So:

• задать вопрос = to ask a question

Russian often uses a noun where English uses just a verb:

• English: ask a question
• Russian: задать вопрос

Why задать and not задавать?

• задать is perfective
• задавать is imperfective

Here, perfective is natural because it refers to asking a question as a complete act, and especially because the sentence means you may ask as many questions as you like, i.e. individual completed questions.

Compare:

• Можно задать вопрос? = May I ask a question?
• Он любит задавать вопросы. = He likes asking questions.
  Here задавать is more natural because it describes a repeated/general activity.

Сколько угодно means:

• as many as you like
• any number you want
• however many you want

So:

• задать сколько угодно вопросов = to ask as many questions as you want

This expression is very common in Russian and can be used with other words too:

• бери сколько угодно = take as much/as many as you want
• говори сколько угодно = say as much as you like
• где угодно = anywhere
• когда угодно = anytime

The word угодно originally has the idea of pleasing / as one wishes, but in modern use it often just forms expressions like whatever / however much / wherever / whenever.

Because after сколько and expressions like сколько угодно, Russian normally uses the genitive plural.

So:

• nominative plural: вопросы
• genitive plural: вопросов

That is why we get:

• сколько угодно вопросов

This is similar to:

• много вопросов = many questions
• несколько вопросов = several questions
• сколько вопросов? = how many questions?

In all of these, the noun is not in nominative plural.

Непонятна is the short form of the adjective непонятный.

Russian often uses the short adjective when saying that something is a certain way:

• тема непонятна = the topic is unclear
• дверь закрыта = the door is closed
• он готов = he is ready

Meanwhile, the long form is more descriptive/attributive:

• непонятная тема = an unclear topic

So:

• тема непонятна = the topic is unclear
• непонятная тема = an unclear topic

Both relate to the same adjective, but the short form is especially common in predicate position.

Because it agrees with тема, which is:

• feminine
• singular

Short-form adjectives still show gender and number:

• masculine: непонятен
• feminine: непонятна
• neuter: непонятно
• plural: непонятны

So:

• тема непонятна = feminine singular
• урок непонятен = masculine singular
• объяснение непонятно = neuter singular
• правила непонятны = plural

Если means if.

So the sentence means:

• You can ask as many questions as you want if the topic is unclear.

This is a real condition: in the case that the topic is unclear.

Could когда work?

• когда means when
• In some contexts, когда can sound similar to if/when, but here если is the normal choice because it expresses a condition, not simply a time.

Compare:

• Если тема непонятна, задавайте вопросы. = If the topic is unclear, ask questions.
• Когда тема станет понятна, вопросов не будет. = When the topic becomes clear, there will be no questions.

Yes. Russian word order is flexible, and the basic meaning usually stays the same.

Original:

• На семинаре можно задать сколько угодно вопросов, если тема непонятна.

Possible variations:

• Если тема непонятна, на семинаре можно задать сколько угодно вопросов.
• Сколько угодно вопросов можно задать на семинаре, если тема непонятна.

The differences are mostly about emphasis and style, not core meaning.

The original order sounds natural and neutral:

1. setting: На семинаре
2. possibility: можно
3. action: задать сколько угодно вопросов
4. condition: если тема непонятна

It can suggest both, and that is very common with можно.

Depending on context, можно may mean:

• it is allowed
• it is possible
• you may
• you can

Here the most natural interpretation is something like:

• At the seminar, you’re free to ask as many questions as you want if the topic is unclear.

So it carries both ideas:

• the seminar format allows it
• the situation makes it possible/appropriate

Russian often leaves this distinction less explicit than English does.

Yes, but it would shift the nuance.

• задать сколько угодно вопросов focuses on asking questions as completed acts
• задавать сколько угодно вопросов focuses more on the ongoing/repeated activity of asking questions

In this sentence, задать sounds more idiomatic and tighter.

Compare:

• На семинаре можно задать сколько угодно вопросов.
  = At the seminar, you can ask as many questions as you want.
• На семинаре можно задавать вопросы.
  = At the seminar, you can ask questions / asking questions is allowed.

With сколько угодно, the perfective задать is especially natural.

Yes: it sounds natural, neutral, and slightly formal/educational.

A few things contribute to that:

• На семинаре sets an academic context
• можно makes it impersonal and generally applicable
• задать вопрос is standard formal-neutral Russian
• если тема непонятна is polite and indirect

It sounds like something a teacher or course organizer might say.

A more conversational version could be:

• Если что-то непонятно, на семинаре можно задавать любые вопросы.

That is a bit looser and more everyday, but the original sentence is perfectly normal and clear.