Моя сестра моложе, чем мой брат.

my

Breakdown of Моя сестра моложе, чем мой брат.

• сестра: the sister
• брат: the brother
• чем: than

Questions & Answers about Моя сестра моложе, чем мой брат.

Where is the word “is” in the Russian sentence? Why is it missing?

Russian normally leaves out the verb “to be” in the present tense when it means “is / am / are.”

So instead of literally saying Моя сестра есть моложе, чем мой брат, Russian just says:

• Моя сестра моложе, чем мой брат.
  → literally: My sister younger than my brother.

The “is” is understood from the structure [subject] + [adjective], so no separate word is needed in the present tense. (In past and future you do use forms of быть: была, будет, etc.)

Why is it моя сестра, not something like моей сестры or мою сестру?

Моя сестра is in the nominative case because it’s the subject of the sentence.

• моя – feminine singular nominative form of мой, agreeing with сестра
• сестра – feminine singular nominative, “sister”

Other forms would be different cases and wrong here:

• моей сестры – genitive (“of my sister”)
• мою сестру – accusative (“my sister” as a direct object)

Since “my sister” is the one who is younger, it must be nominative: моя сестра.

What exactly is моложе grammatically? Why doesn’t it change for gender or number?