Мой брат хмурится, когда не понимает новое правило.

Because брат is a masculine singular noun. In Russian, possessive words like мой must agree with the noun they describe.

• masculine: мой брат
• feminine: моя сестра
• neuter: моё правило
• plural: мои друзья

So мой брат is the correct match.

Russian does not use articles. There is no direct equivalent of English a/an or the.

So брат can mean:

• brother
• a brother
• the brother
• my brother if a possessive is added

The exact meaning comes from context. In this sentence, мой брат is naturally understood as my brother.

Хмурится means something like:

• frowns
• scowls
• looks gloomy
• makes a displeased expression

So it describes your brother’s facial expression or mood when he does not understand something.

It is the 3rd person singular present-tense form of хмуриться.

The dictionary form is хмуриться, and the -ся is part of the verb.

Many Russian verbs are reflexive or have a reflexive-looking ending even when English does not use himself or oneself. Here, хмуриться is just the normal verb meaning to frown / to look sullen.

So you should learn хмуриться as a whole word, not as хмурить plus an optional ending.

Because the subject is already clear from context.

The sentence means:

• My brother frowns when he doesn't understand a new rule.

In Russian, subject pronouns like он are often omitted if the meaning is obvious. The verb form не понимает already tells you it is he/she in the present tense, and here we know it refers to мой брат.

A version with он is possible, but it is less natural unless you want extra emphasis:

• Мой брат хмурится, когда он не понимает новое правило.

Because this sentence describes a habitual or general situation.

It is not talking about one single moment only. It means something like:

• My brother frowns whenever he doesn't understand a new rule.

Russian uses the present tense for this kind of repeated, regular behavior, just like English does in sentences such as He gets upset when he loses.

This is about aspect.

• понимает comes from понимать — imperfective
• поймёт comes from понять — perfective

Here the sentence describes a repeated or general situation, so the imperfective verb is the natural choice:

• когда не понимает = when/whenever he doesn't understand

If you used не поймёт, it would sound more like a single future occasion:

• when he fails to understand
• when he won't understand

So не понимает is right for a general habit.

It is in the accusative case, because it is the direct object of не понимает.

He does not understand what?

• новое правило

But there is an important detail: for neuter singular inanimate nouns, the accusative looks the same as the nominative.

So:

• nominative: новое правило
• accusative: новое правило

The adjective новое agrees with правило in:

• gender: neuter
• number: singular
• case: accusative

A very common clue is the ending.

Many Russian nouns ending in -о or -е are neuter, and правило is one of them.

Because правило is neuter, the adjective must also be neuter:

• новый for masculine
• новая for feminine
• новое for neuter

So:

• новое правило = a new rule

Because когда не понимает новое правило is a subordinate clause.

Russian punctuation normally requires a comma before subordinate conjunctions such as:

• когда = when
• если = if
• потому что = because
• что = that

So the comma in this sentence is mandatory:

• Мой брат хмурится, когда не понимает новое правило.

Literally, когда means when, but in a sentence like this it often has the sense of whenever.

So the sentence can be understood as:

• My brother frowns when he doesn't understand a new rule
• My brother frowns whenever he doesn't understand a new rule

The present-tense, habitual meaning creates that whenever feeling.

Yes. Russian word order is more flexible than English word order.

The original sentence is neutral and natural:

• Мой брат хмурится, когда не понимает новое правило.

You could also say:

• Когда мой брат не понимает новое правило, он хмурится.

This has the same basic meaning, but the emphasis shifts slightly because the when-clause comes first.

Russian word order often changes for focus, style, or emphasis, not because the grammar changes.

Sometimes yes, if the context already makes it clear whose brother you mean.

For example, in conversation, Брат хмурится... might be enough if everyone already knows you are talking about your brother.

But мой брат is perfectly normal and often clearer, especially when introducing the subject or contrasting him with someone else.