Grammar

[Shatnerfan]

When to use elder/older and eldest/oldest?

[Sylday]

Use "elder/older" when there are only two brothers; "eldest/oldest" when there are more than two.

[Shatnerfan]

To clarify, I mean what's the difference between elder and older and what's the difference between oldest and eldest?

[Quizmonster]

The pairs of words are effectively synonymous. There is no real difference in basic meaning, though usage varies. As Sylday says, 'elder' refers to a comparison of two specific people - usually siblings - whilst 'eldest' refers to three or more.

There are other situations where elder/older are not exchangeable...eg I might say: "I'm older than you"...not "I'm elder than you" etc. Also 'elder', used as a noun, means someone of age and respect within a community.