Мы обсудили последствия решения.

Yes, you often can leave it out.

Russian verb endings usually show who did the action, and обсудили already tells you the subject is we because it is past plural. So:

• Мы обсудили последствия решения.
• Обсудили последствия решения.

Both can mean We discussed the consequences of the decision.

Including мы makes the subject more explicit. It can also add slight emphasis, as in we discussed it.

This is an aspect question.

• обсудили = perfective
• обсуждали = imperfective

обсудили presents the discussion as a completed whole: we had the discussion and finished it.

обсуждали would focus more on the process or the fact that it was going on, without necessarily saying it reached completion.

So:

• Мы обсудили последствия решения. = We discussed them and completed that discussion.
• Мы обсуждали последствия решения. = We were discussing / used to discuss the consequences.

обсудили is the past tense plural form of обсудить.

Breakdown:

• infinitive: обсудить
• stem: обсуди-
• past plural ending: -ли

So:

• masculine singular: обсудил
• feminine singular: обсудила
• neuter singular: обсудило
• plural: обсудили

Because the subject is мы, the plural form is used.

Yes, последствия is plural here.

The singular noun is:

• последствие = consequence

The plural is:

• последствия = consequences

In this sentence, последствия is the direct object of обсудили, so it is in the accusative plural. But because it is an inanimate noun, the accusative plural looks the same as the nominative plural:

• nominative plural: последствия
• accusative plural: последствия

So there is no visible change.

решения is genitive singular of решение.

• nominative singular: решение
• genitive singular: решения

Here it means of the decision.

So:

• последствия решения = the consequences of the decision

Russian often uses the genitive case where English uses of.

Because Russian usually expresses that relationship with a case ending, not with a separate word.

In English:

• consequences of the decision

In Russian:

• последствия решения

The ending -я on решения shows the relationship. This is one of the most common uses of the genitive case.

Not always.

решение can mean:

• decision
• solution
• sometimes resolution, depending on context

So последствия решения could mean:

• consequences of the decision
• consequences of the solution

In this sentence, context usually tells you which one is meant.

The given order is very normal:

• Мы обсудили последствия решения.

It is a neutral subject–verb–object order.

But Russian word order is more flexible than English. You can change it for emphasis or style, for example:

• Последствия решения мы обсудили.
  This emphasizes the consequences of the decision.

• Обсудили мы последствия решения.
  This can sound more marked or contrastive, depending on context.

So the original sentence is the safest and most neutral version.

The stress is:

• мы
• обсуди́ли
• после́дствия
• реше́ния

So the whole sentence is pronounced roughly like:

• мы обсуди́ли после́дствия реше́ния

Stress matters in Russian, so it is worth learning words together with their stress.

Russian has no articles.

So Russian does not have separate words matching English a/an and the. Whether something is the decision, a decision, or just decision is understood from context.

That is why:

• решения can mean of the decision or of a decision, depending on context.

In many sentences, the broader situation makes it clear which interpretation is intended.

Sometimes, yes, depending on context.

Russian past tense does not directly map onto just one English tense. Обсудили is past, but in translation it can correspond to:

• We discussed the consequences of the decision
• We have discussed the consequences of the decision

The exact English choice depends on context and style. The Russian sentence itself mainly tells you that the discussion happened and was completed.