Он поделился своим горем с другом.

The dictionary form is поделиться.

In this sentence, поделился is:

• past tense
• masculine singular
• perfective

So it agrees with он and means that the action happened as a completed event: he shared.

A useful pattern to remember is:

поделиться чем? с кем?
= to share something with someone

So in this sentence:

• горем = what he shared
• с другом = with whom he shared it

The -ся ending is part of the verb поделиться. It is a reflexive form, but here it does not mean literally himself in the English sense. Instead, it is simply how this verb is normally used.

Russian often has verbs with -ся that function as standard dictionary verbs. In this case:

• делить = to divide
• поделиться = to share

So you should learn поделиться as a whole verb, not try to translate -ся separately every time.

This is mainly a question of aspect.

• поделиться = perfective → a completed act of sharing
• делиться = imperfective → sharing in general, repeatedly, or as a process

So:

• Он поделился своим горем с другом
  = He shared his grief with a friend / He opened up to a friend.

But:

• Он делился своим горем с другом
  suggests an ongoing or repeated action, such as He was sharing his grief with a friend or He used to share his grief with a friend.

Russian often uses свой when the possession belongs to the subject of the sentence.

Here, the subject is он, and the grief is his own, so Russian prefers:

• своим горем = his own grief

Using его горем would usually sound less natural here and could even create ambiguity, as его might refer to someone else’s grief depending on context.

So a very important rule is:

• when the possessor is the subject, Russian often uses свой

Because it has to agree with горем.

The noun горе is:

• neuter
• singular

And after поделиться, the thing being shared goes in the instrumental case. So горе becomes горем.

The possessive adjective must match that noun in:

• gender
• number
• case

So:

• nominative: своё горе
• instrumental: своим горем

Because the verb поделиться requires the thing shared to be in the instrumental case.

The pattern is:

поделиться чем?
= to share what?

So:

• горе → nominative/dictionary form
• горем → instrumental form

This is just the normal case pattern of the verb. You can compare:

• поделиться новостью = to share news
• поделиться идеей = to share an idea
• поделиться опытом = to share experience

Because Russian expresses with whom using с + instrumental.

The pattern is:

поделиться чем? с кем?

So:

• друг = friend
• с другом = with a friend / with the friend

This is another instrumental form:

• друг → другом

So the sentence has two instrumental phrases:

• своим горем = what he shared
• с другом = with whom he shared it

It can mean either, depending on context.

Russian has no articles, so другом does not by itself tell you whether the meaning is:

• a friend
• the friend
• sometimes even his friend, if the context makes that clear

English has to choose an article, but Russian often leaves that information to context.

Yes, горе is quite a strong word. It means grief, sorrow, or deep sadness.

It sounds more serious than just:

• печаль = sadness
• грусть = melancholy / sadness

So поделиться своим горем suggests that he told someone about something painful and emotionally heavy.

Yes. Russian word order is flexible, although the original sentence is the most neutral:

• Он поделился своим горем с другом.

Other orders are possible, for example:

• С другом он поделился своим горем.
• Своим горем он поделился с другом.

The basic meaning stays the same, but the emphasis changes:

• putting с другом earlier can highlight with whom
• putting своим горем earlier can highlight what he shared

Russian often uses word order for emphasis rather than for basic grammar.

Because поделиться is perfective, it does not normally form a true present-tense meaning. Its present-looking forms actually refer to the future.

So:

• past: Он поделился своим горем с другом.
  = He shared his grief with a friend.

• future: Он поделится своим горем с другом.
  = He will share his grief with a friend.

If you want a true present meaning such as He is sharing or He shares, you would normally use the imperfective verb:

• Он делится своим горем с другом.

In the past tense, Russian verbs show gender in the singular.

Compare:

• он поделился = masculine
• она поделилась = feminine
• оно поделилось = neuter
• они поделились = plural

So the ending -лся tells you it is masculine singular, matching он.