Мы обсудили все обстоятельства.

Обсудили is the past tense plural form of the verb обсудить, which means to discuss in the sense of to discuss something through / to finish discussing it.

Because the subject is мы (we), the verb has to be in the plural past form.

A useful pattern:

• обсудить = infinitive
• я обсудил / обсудила = I discussed
• мы обсудили = we discussed

In the past tense, Russian verbs show number, and in the singular they also show gender.

This is a question of aspect, which is very important in Russian.

• обсудили = perfective
• обсуждали = imperfective

Here, обсудили means the discussion is seen as completed:
We discussed all the circumstances / We have discussed all the circumstances.

If you said Мы обсуждали все обстоятельства, that would focus more on the process of discussing, not necessarily the result. It can mean something like:

• We were discussing all the circumstances
• We discussed all the circumstances (but without stressing completion)
• We used to discuss all the circumstances in the right context

Because the sentence includes все обстоятельства (all the circumstances), the perfective обсудили sounds very natural: it suggests that everything relevant was covered.

It can mean either, depending on context.

Russian does not have a separate tense formed with have, the way English does. So:

• Мы обсудили все обстоятельства can mean We discussed all the circumstances
• or We have discussed all the circumstances

The perfective verb often gives a completed-result feeling, which is why We have discussed... is often a good translation in the right context.

Обстоятельства is in the accusative plural, because it is the direct object of обсудили.

The verb обсудить takes a direct object:

• обсудить вопрос = to discuss a question
• обсудить проблему = to discuss a problem
• обсудить обстоятельства = to discuss the circumstances

Why does обстоятельства look the same as the nominative plural? Because it is an inanimate plural noun, and for inanimate nouns in Russian:

• nominative plural = accusative plural

So:

• nominative: обстоятельства
• accusative: обстоятельства

Same form, different function.

Because обстоятельства is plural.

• все = all (plural) / everyone
• всё = everything / all of it (neuter singular)

So:

• все обстоятельства = all the circumstances
• всё by itself = everything

Compare:

• Мы обсудили все обстоятельства. = We discussed all the circumstances.
• Мы обсудили всё. = We discussed everything.

This is a very common distinction for learners.

Here обстоятельства means something like:

• circumstances
• surrounding facts
• details of the situation
• relevant factors

So the sentence suggests that all the important details or facts connected with the situation were discussed.

The singular is обстоятельство = circumstance.

Also, you may see обстоятельство in grammar books, where it means adverbial modifier, but that is a completely different use. In this sentence, it has the ordinary everyday meaning.

Yes, sometimes — but not always as naturally as in some other cases.

Russian often drops subject pronouns when the subject is clear from context. However, with the past tense plural, the verb form обсудили by itself does not tell you whether the subject is:

• we
• they
• you (plural or formal)

So мы is helpful because it makes the subject explicit.

Without context:

• Обсудили все обстоятельства. could mean We discussed..., They discussed..., or even You discussed...

So in this sentence, мы is quite useful.

The word order is not completely fixed.

The neutral, standard order here is:

• Мы обсудили все обстоятельства.

But Russian word order can change for emphasis or information structure.

For example:

• Все обстоятельства мы обсудили.
  This emphasizes all the circumstances — as if contrasting them with something else.

• Обсудили мы все обстоятельства.
  This is more marked and might sound contrastive, depending on context.

So the original sentence is the most neutral way to say it.

The main stress is:

Мы обсуди́ли все обстоя́тельства.

A rough English-friendly guide:

mɨ ob-soo-DEE-lee fsye ab-sta-YA-tyel-stva

A few notes:

• обсуди́ли has the stress on -ди-
• обстоя́тельства has the stress on -я́-
• unstressed о in Russian is often pronounced more like a, so the beginning of обстоятельства may sound closer to ab-

In Russian, все + plural noun usually means all the ....

So:

• все обстоятельства = all the circumstances
• все книги = all the books
• все вопросы = all the questions

If you want the idea of every in a more distributive sense, Russian often uses forms like каждый / каждая / каждое:

• каждое обстоятельство = every circumstance

So in this sentence, все обстоятельства means the whole set of relevant circumstances.