Questions & Answers about Seine Freundin ist sehr jung.
Why is it seine and not just sein?
The base word for "his" is sein. Because the noun that follows it (Freundin) is feminine, the possessive word must take the feminine ending -e, making it seine.
If Freundin is feminine, why use the masculine root sein- instead of ihr-?
The root of the possessive word depends on the owner. Since the owner is "he", we use sein- (his). The ending depends on the person being owned. Since Freundin is feminine, we add -e. If the sentence meant "her girlfriend", it would be ihre Freundin.
How do we know Freundin means "girlfriend" here and not just "female friend"?
In German, using a direct possessive word like "his friend" (seine Freundin) or "my friend" (mein Freund) almost always implies a romantic relationship. To say "a female friend of his" without romantic meaning, Germans usually say eine Freundin von ihm.
Since Freundin is feminine, why doesn't the adjective have an ending like junge?
In German, adjectives only get endings when they are placed directly before a noun (like seine junge Freundin). When an adjective comes after the verb to describe the noun, it stays in its basic, unchanged dictionary form: jung.