Его поведение изменилось.

Here его means his.

In Russian, его can mean either:

• him as an object: Я вижу его = I see him
• his as a possessive: его поведение = his behavior

In this sentence, его is possessive because it modifies the noun поведение.

So:

• его поведение = his behavior

Because он means he, not his.

Russian personal pronouns change form depending on their role:

• он = he
• его = him / his

So:

• он изменился = he changed
• его поведение изменилось = his behavior changed

You need его here because you are showing possession.

Поведение is in the nominative singular, because it is the subject of the sentence.

The thing that changed is behavior, so поведение is the grammatical subject:

• Его поведение = the thing doing the changing
• изменилось = changed

A good literal breakdown is:

• его = his
• поведение = behavior
• изменилось = changed

Because the subject поведение is neuter singular, and the verb is in the past tense.

In Russian past tense, verbs agree with gender and number:

• masculine: изменился
• feminine: изменилась
• neuter: изменилось
• plural: изменились

Since поведение is a neuter noun, the verb must also be neuter:

• поведение изменилось

This is one of the big differences from English: in the Russian past tense, the verb shows gender/number.

Because it ends in -е, which is a very common neuter noun ending in Russian.

So:

• поведение = neuter singular
• therefore: изменилось

Many abstract nouns ending in -ие are neuter, for example:

• мнение = opinion
• здание = building
• решение = decision
• поведение = behavior

The dictionary form is измениться.

That is the perfective infinitive, meaning to change or to become different.

So:

• измениться = to change, to undergo a change
• изменилось = changed

In this sentence, the idea is that the behavior became different.

Because изменилось comes from измениться, which is used when something changes itself / undergoes change.

Compare:

• изменить что-то = to change something
• измениться = to change

Examples:

• Он изменил своё поведение. = He changed his behavior.
• Его поведение изменилось. = His behavior changed.

So in your sentence, behavior is not changing something else; behavior itself is what changed.

The verb behind изменилось is измениться, which includes the reflexive ending -ся.

In many cases, this reflexive ending helps create an intransitive meaning:

• изменить = to change something
• измениться = to change

So here the reflexive form is what gives the meaning changed rather than changed something.

In the past neuter form, -ся becomes -сь / -лось in spelling and pronunciation patterns, giving изменилось.

Because Russian does not use English-style helping verbs in the same way here.

English may say:

• His behavior changed
• His behavior has changed

Russian simply uses the past tense form:

• Его поведение изменилось

This can often cover both English ideas depending on context.

Изменилось is perfective.

That means the sentence presents the change as a completed event or a result:

• Его поведение изменилось. = His behavior changed / has changed

If you wanted an imperfective idea, you would usually use a different verb form, for example something like:

• Его поведение менялось. = His behavior was changing / used to change
• Его поведение изменялось. = a more formal imperfective version, also was changing

So the perfective form here focuses on the fact that the change happened and the result now exists.

Yes. Russian word order is flexible, though the neutral order here is:

• Его поведение изменилось.

Other orders are possible, for example:

• Поведение его изменилось.
• Изменилось его поведение.

These can sound more marked or emphasize different parts of the sentence.

For a learner, the safest and most neutral version is:

• Его поведение изменилось.

Yes, it would be different.

• его = his
• её = her
• их = their

So:

• Его поведение изменилось. = His behavior changed.
• Её поведение изменилось. = Her behavior changed.
• Их поведение изменилось. = Their behavior changed.

Unlike adjectives, these possessive pronoun forms do not change to match the noun’s gender in this use.

Because его here is a possessive pronoun form that is indeclinable in this function.

That means it stays the same regardless of the gender of the noun being possessed:

• его брат = his brother
• его сестра = his sister
• его поведение = his behavior

So even though поведение is neuter, его still remains его.

Not in this sentence.

Свой is a reflexive possessive meaning one’s own, and it normally refers back to the subject of the sentence.

But in:

• Его поведение изменилось

the subject is поведение (behavior), not he.

So своё поведение изменилось would not work here.

However, in a sentence where he is the subject, свой is natural:

• Он изменил своё поведение. = He changed his behavior.

That is because он is the subject, and своё refers back to он.

A helpful pronunciation guide is:

• Его → roughly yivo
• поведение → pava-DYE-ni-ye
• изменилось → izmye-NEE-las'

The main stresses are:

• его́
• поведе́ние
• измени́лось

A few useful notes:

• его is usually pronounced more like ево / yivo in normal speech
• unstressed о often sounds closer to a
• the stress in измени́лось is on ни

Sometimes, depending on context.

The noun поведение most directly means behavior or conduct. In some contexts, especially if you are talking about how someone acts socially, English might translate it as:

• behavior
• conduct
• sometimes even manner of behaving

But behavior is the most straightforward translation.

A very literal breakdown is:

• Его = his
• поведение = behavior
• изменилось = changed

So the sentence is literally:

• His behavior changed.

Depending on context, natural English could also be:

• His behavior has changed.