зашли русские=) Ответить тоже по-русски? вобщем, В английском языке некоторые прилагательные могут быть только определениями при существительных и не встречаются в позиции именной части сказуемого, вот и наше elder/eldest входит в эту группу (а, как сказуемое, соответственно, используются older/oldest) (Например, His elder sister is a teacher, НО She's five years older than him)

That is the difference between the comparative form of the adjective "old" (older, elder) and the superlative form of the adjective ( the oldest, the eldest). The comparative form is comparing 2 subjects ,where the first subject has relatively more of the quality ,the subject of comparison ,than the second subejct. "He is older than his brother" The superlative form is considering a subject the optimal,when it comes to possessing the qualities of a certain adjective. "The is the oldest man among his friends." "Elder" could mean a senior member, higher in rank or position and also born earlier ( compare with older here). 'He is the eldest among his brothers." ( born first) 'He is elder than his brother Balashov,but younger than his sister Natalie.