[ابو بيشو]

The superlative of old is oldest
That house over there is the oldest building in the town.
We use eldest when we are talking about the members of a family:
My eldest son is 13 years old.
Are you the eldest in your family?
After superlatives, we use in with places (towns,buildings
What's the longest river in the world?
the best in the class/the team/the company
لكن
the happiest day of my life, the hottest day of the year
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We use elder when we are talking about members of a family:comparative
My elder brother is a pilot.
We use elder only before a noun:
My brother is older than me.(not elder than me)
شكرا لكم