Мы обсудили причину его отсутствия.

Обсудили is the past tense plural form of обсудить.

It tells you three things at once:

• обсудить is the verb
• past tense: the discussion already happened
• plural: the subject is мы (we)

It is also perfective, so it presents the action as completed: we discussed it / we finished discussing it.

Because причину is the accusative singular form of причина.

Here, причину is the direct object of обсудили — it is the thing that we discussed.

For many feminine nouns ending in -а, the accusative singular changes to -у:

• причина → причину
• машина → машину
• проблема → проблему

So обсудили причину = discussed the reason.

Because it is in the genitive singular.

The noun причина often takes another noun in the genitive to mean the reason for/of something:

• причина отсутствия = the reason for the absence
• причина ошибки = the reason for the mistake
• причина задержки = the reason for the delay

The base form is отсутствие (absence), and here it becomes отсутствия.

So the structure is:

• причину = the reason
• его отсутствия = of/for his absence

Here его means his.

So его отсутствия means his absence.

A common point of confusion is that его can also mean him in other contexts. But when it comes directly before a noun, as it does here, it is usually a possessive word:

• его книга = his book
• его идея = his idea
• его отсутствие = his absence

So in this sentence, его is not him — it is his.

Because его as a possessive word is indeclinable in modern Russian.

That means it stays the same regardless of the gender, number, or case of the noun after it:

• его брат = his brother
• его сестра = his sister
• его друзья = his friends
• без его брата = without his brother
• причина его отсутствия = the reason for his absence

This is different from words like мой, твой, наш, which do change form.

Because свой refers back to the subject of the sentence.

The subject here is мы (we), so своего отсутствия would mean something like our own absence, not his absence.

Since the absence belongs to some other male person, Russian uses его, not свой.

Compare:

• Мы обсудили его отсутствие. = We discussed his absence.
• Мы обсудили своё отсутствие. = We discussed our own absence.

So его is the correct choice here.

Russian often uses the genitive case where English uses a preposition like of or for.

So:

• причина отсутствия literally looks like reason of absence
• but in natural English it means reason for the absence

This is very normal in Russian. You should think of причина + genitive as a standard pattern.

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

The neutral order here is:

• Мы обсудили причину его отсутствия.

But you could also say:

• Причину его отсутствия мы обсудили.

That puts more emphasis on the reason for his absence.

Even though word order can change, not every order sounds equally natural. The version you were given is the most neutral and standard.

Because Russian has no articles like a or the.

So причину can mean:

• a reason
• the reason

The exact meaning depends on context.

In this sentence, English usually translates it as the reason because the situation sounds specific, but Russian itself does not use a separate word for the.

Yes, but the meaning changes a little.

• Мы обсудили причину его отсутствия.
  = We discussed the reason for his absence, and the discussion is presented as completed.

• Мы обсуждали причину его отсутствия.
  = We were discussing / discussed the reason for his absence, with more focus on the process and not necessarily on completion.

So both are possible, but обсудили is better if you want to present it as a finished discussion.

In standard Russian, его is usually pronounced roughly like yivo / ivo, not yego.

So in this sentence, learners usually hear something close to:

• мы обсудили причину иво отсутствия

This is a normal pronunciation pattern in modern Russian. You just have to learn it as a common exception-like feature of words such as его.

So:

• spelling: его
• pronunciation: roughly yivo