Learn Farsi: حیوان‌هایی که در این رودخانه بوده‌اند، چند میلیون سال پیش منقرض شده‌اند. - The animals that were in this river went extinct several million years ago.

Why does the Farsi sentence use the Present Perfect tense (بوده‌اند and منقرض شده‌اند) instead of the Simple Past, since this happened millions of years ago?

In English, we use the Simple Past ('went extinct') for ancient events. However, Farsi uses the Present Perfect to talk about past events the speaker didn't personally witness. Because nobody was alive millions of years ago to see these animals, Farsi uses the Present Perfect to show this is a reported historical fact or scientific discovery rather than a firsthand memory.

Why is there an extra یی at the end of حیوان‌هایی (animals)?

That extra sound is the restrictive ی which acts as a bridge before the relative pronoun که (that/which). It specifically points out that we are talking about 'the animals THAT were in this river,' not just animals in general. Because the plural suffix ها ends in a vowel, Farsi adds an extra consonant ی before the restrictive ی, creating the یی ending.

If there are millions of years, why is the word سال (year) in its singular form?

In Farsi grammar, any noun that directly follows a number or a quantity word like چند (several) must remain singular. So instead of 'several million years,' the literal Farsi structure is 'several million year'.

How is the verb for 'went extinct' formed in this sentence?

It uses the compound verb منقرض شدن (to become extinct). To make it Present Perfect, only the second part of the verb changes. The word stays exactly the same, and (to become) turns into (they have become).