Learn Turkish: İki ortak var, hangisi daha başarılı? - There are two partners, which one is more successful?

hangi is an adjective meaning "which" (as in hangi ortak - "which partner"). By adding the third-person possessive suffix -(s)i, we turn it into the pronoun hangisi ("which one" or "which of them"). The -si links directly back to the partners mentioned earlier.

Since there are two partners, why isn't it İki ortaklar?

In Turkish, nouns always stay in their singular form when they come directly after a number or a quantity word. So you must say iki ortak (literally "two partner"), never iki ortaklar.

How is the word başarılı formed?

It is made from the noun başarı (success) plus the suffix -lı, which means "with" or "having". Therefore, başarılı literally means "having success," which is how Turkish expresses the adjective "successful."

Where is the verb for "is" in hangisi daha başarılı?

In everyday Turkish, the third-person "to be" verb (is/are) is often completely invisible. You could add the formal suffix -dır and say başarılıdır, but simply ending with the adjective başarılı is perfectly complete and much more common.