Сессия вопросов и ответов уже закончилась.

Because сессия is the subject of the sentence: it is the thing that ended.

In Сессия вопросов и ответов уже закончилась, the basic structure is:

• сессия = the subject
• закончилась = ended / came to an end

So сессия stays in the nominative singular.

They depend on сессия and specify what kind of session it was: a session of questions and answers.

So:

• вопрос → вопросов = of questions
• ответ → ответов = of answers

This is a very common Russian pattern: a noun followed by another noun in the genitive to show a relationship similar to English of.

So literally:

• сессия вопросов и ответов = session of questions and answers

In natural English, we often say Q&A session.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject is сессия, and сессия is a feminine singular noun. So the past tense verb must also be feminine singular:

• masculine: закончился
• feminine: закончилась
• neuter: закончилось
• plural: закончились

That is why the sentence uses закончилась.

The verb here is закончиться, not закончить.

Compare:

• закончить что-то = to finish something
  
  • Он закончил работу. = He finished the work.
• закончиться = to come to an end / to be over
  
  • Сессия закончилась. = The session ended.

So the -ся / -сь form often makes the verb intransitive here: the subject itself comes to an end, rather than someone actively finishing an object.

The -сь spelling is used instead of -ся because it comes after a vowel in закончилась.

This is a question of aspect.

• закончилась = perfective
  Focuses on the completed result: it ended
• заканчивалась = imperfective
  Focuses on the process or background: it was ending / was coming to an end

In this sentence, the speaker means the Q&A session is already over, so the completed idea is needed. That is why закончилась is the natural choice.

Уже usually means already.

So:

• Сессия вопросов и ответов закончилась. = The Q&A session ended.
• Сессия вопросов и ответов уже закончилась. = The Q&A session has already ended.

It adds the idea that this happened before now, or perhaps sooner than expected, depending on context.

Yes. Russian word order is flexible, and уже can move around for emphasis.

For example:

• Сессия вопросов и ответов уже закончилась.
• Уже закончилась сессия вопросов и ответов.
• Сессия уже закончилась.

These all mean roughly the same thing, but the emphasis shifts slightly.

The version you were given is neutral and natural.

Yes, but it is not exactly the same in tone.

• Сессия вопросов и ответов уже закончилась.
  = The Q&A session has already ended.
  This describes an event that came to an end.

• Сессия вопросов и ответов уже закончена.
  = The Q&A session is already finished.
  This sounds more like a resulting state.

Both are possible, but закончилась is often more natural when talking about an event ending.

Yes, it is a normal and understandable phrase.

A few possibilities in Russian are:

• сессия вопросов и ответов
• сеанс вопросов и ответов
• вопросы и ответы in some contexts
• sometimes even Q&A in modern, especially business or online contexts

Of these, сессия вопросов и ответов is a clear, standard translation of Q&A session.

Sometimes yes, depending on context.

Russian закончилась literally means ended / came to an end, but in natural English the best translation may vary:

• The Q&A session ended.
• The Q&A session was over.
• The Q&A session had already finished.

If the meaning has already been given, the important grammar point is that закончилась is a completed past event.

Russian does not form the present perfect the way English does.

English:

• has ended

Russian:

• usually just the past tense form, often with context words like уже

So:

• уже закончилась can correspond to English has already ended

Russian expresses this meaning without a separate auxiliary verb like has.

The dictionary form is закончиться.

To break it down:

• prefix: за-
• root related to конец = end
• infinitive ending: -ть
• reflexive marker: -ся

Then in the past tense feminine singular, it becomes:

• закончилась

So when looking it up in a dictionary, search for закончиться.

A helpful stress pattern is:

• сЕссия
• вопрОсов
• отвЕтов
• ужЕ
• закончИлась

So the whole sentence is pronounced approximately as:

СЕссия вопрОсов и отвЕтов ужЕ закончИлась.

A few pronunciation notes:

• сессия has a soft сс
• вопросов has stress on -ро-
• ответов has stress on -ве-
• закончилась has stress on -чи-

Yes.

You can say:

• Сессия уже закончилась. = The session has already ended.

That is fully grammatical.
Adding вопросов и ответов simply makes it more specific: it tells you what kind of session it was.

So the longer phrase is:

• main noun: сессия
• defining genitive phrase: вопросов и ответов