Learn Hardcore Russian: Её брат, мой дядя, моложе: ему сорок лет. - Her brother, my uncle, is younger: he is forty years old.

Её is a possessive pronoun meaning her (as in her brother). This word is indeclinable: it never changes its form for case, gender, or number. It is always её:

её брат – her brother
я видел её брата – I saw her brother
я говорил с её братом – I spoke with her brother

Only the noun (брат, брата, братом…) changes for case, not её. In your sentence, we need nominative брат because we are stating who is younger: Её брат … моложе.

What is the difference between её and она?

Она = she. It is a subject pronoun, used as the subject of a sentence:
- Она моложе. – She is younger.
Её = her. In this sentence it is a possessive pronoun (her brother). It can also be the object pronoun her:
- Я люблю её. – I love her.
- её брат – her brother.

So you use она when she is the one doing or being something, and её when you mean her as a possessor (her X) or an object (I see her).

Why are there commas around мой дядя?

Мой дядя is an apposition: it renames and explains её брат.

The structure is like English:

Её брат, мой дядя, моложе…
Her brother, my uncle, is younger…

Commas mark this extra identifying information. If you remove мой дядя, the sentence is still grammatically complete:

Её брат моложе: ему сорок лет. – Her brother is younger: he is 40.

So the commas show that мой дядя is additional, explanatory information about её брат.

Why isn’t there a word for is before моложе?

Russian usually drops the present tense of “to be” (быть) in sentences like X is Y.

English: Her brother is younger.
Russian: Её брат моложе. (literally: Her brother younger.)
The “is” is understood from context.

The verb быть appears in:

past: Её брат был моложе. – Her brother was younger.
future: Её брат будет моложе. – Her brother will be younger.

But in the present, it is normally omitted: Её брат моложе.

What exactly is моложе? Does it change for gender or number?

Моложе is the comparative form of the adjective молодой (young). It means younger.

Key points:

It is a short comparative form.
It does not change for gender, number, or case:
- Он моложе. – He is younger.
- Она моложе. – She is younger.
- Они моложе. – They are younger.

So you don’t say молодая / молодой / молодое here; you always use моложе when you mean younger in a comparison.

If you need to specify than whom, you add чем or use the genitive:

Он моложе меня. – He is younger than me.
Он моложе, чем я. – He is younger than I am.

Why is there a colon before ему сорок лет? Could I use something else?

The colon introduces an explanation or clarification of what comes before:

Её брат, мой дядя, моложе: ему сорок лет.
= Her brother, my uncle, is younger: (namely) he is 40.

Typical alternatives:

Dash:
Её брат, мой дядя, моложе — ему сорок лет. (very natural in modern Russian)
Period:
Её брат, мой дядя, моложе. Ему сорок лет.

A comma instead of the colon or dash here would be considered a punctuation error in standard written Russian. So use colon, dash, or period to separate the explanation ему сорок лет.

How does ему сорок лет literally work, and why is ему in the dative case?

Literally, ему сорок лет is:

ему – to him (dative case of он)
сорок – forty
лет – years (genitive plural of год)

So the structure is “To him [there are] forty years” → He is forty (years old).

In Russian, age is usually expressed with dative + number + год/года/лет:

Мне двадцать лет. – I am 20.
Ей тридцать два года. – She is 32.
Им пять лет. – They are 5.

That’s why ему is dative: you’re literally saying to him there are forty years.

Why is it сорок лет, not сорока лет or сорок года?

Two separate points:

Forms of “сорок” (40)
- Nominative/Accusative: сорок
- All other cases: сорока
In ему сорок лет, the numeral is in the nominative as part of the predicate (сорок is what he “has” / what his age is), so you use сорок, not сорока.
Forms of “год” (year) after numbers
- 1 → год: ему один год – he is 1 year old
- 2–4 → года: ему три года – he is 3 years old
- 5–20 → лет: ему пятнадцать лет – he is 15 years old
- 21 → год again, 22–24 → года, etc.

With сорок (40), you use лет:

ему сорок лет – he is 40 years old.

So the correct combination is сорок лет.

Could I say он сорок лет instead of ему сорок лет?

No, он сорок лет is not correct Russian for expressing age.

You must use one of the standard age patterns:

Ему сорок лет. – He is 40 (years old).
Ему исполнилось сорок лет. – He turned 40.
Ему уже сорок лет. – He is already 40.

Using он directly with the number (like он сорок лет) sounds ungrammatical. Age in Russian is expressed with dative (кому? – ему) + number + год/года/лет, not with nominative он + number.

If the person were female (my aunt), what would change in the sentence?

You would change the nouns and the dative pronoun for a woman, but моложе stays the same:

Её сестра, моя тётя, моложе: ей сорок лет.
- Её сестра – her sister
- моя тётя – my aunt
- ей сорок лет – she is 40 (dative of она is ей)

Note:

моложе is unchanged (comparative doesn’t mark gender/number).
Only the nouns (брат → сестра, дядя → тётя) and the dative pronoun (ему → ей) change.

How do you pronounce the main words here, and where is the stress?

Stress and rough pronunciation:

Её – [йи‑ЙО] (stress on the second syllable: еЁ)
брат – [brat] (single syllable, stressed)
мой – [moy] (stressed)
дядя – [ДЯ‑дʹа], stress on ДЯ
моложе – [ma‑ЛО‑zhэ], stress on ЛО
ему – [йи‑МУ], stress on МУ
сорок – [СО‑ruk], stress on СО
лет – [lyet], stressed (single syllable)

So the natural rhythm is:
ЕЁ брат, МОЙ ДЯдя, моЛОже: еМУ СОрок ЛЕТ.