frontier [ˈfrʌntɪə], ancestor [ˈænsəstə], use [ju:s]

"That is Thak," answered the priest, caressing his temple. "Some would call him an ape, but he is almost as different from a real ape as he is different from a real man. His people dwell far to the east, in the mountains that fringe the eastern frontiers of Zamora. There are not many of them; but, if they are not exterminated, I believe they will become human beings in perhaps a hundred thousand years. They are in the formative stage; they are neither apes, as their remote ancestors were, nor men, as their remote descendants may be. They dwell in the high crags of well-nigh inaccessible mountains, knowing nothing of fire or the making of shelter or garments, or the use of weapons. Yet they have a language of a sort, consisting mainly of grunts and clicks.

"I took Thak when he was a cub (я взял Така, когда он был детенышем; to take), and he learned what I taught him much more swiftly and thoroughly (и он учился тому, чему я его учил гораздо быстрее и основательнее; to teach — обучать, преподавать; thoroughly — полностью; основательно) than any true animal could have done (чем могло бы /сделать/ любое истинное животное; to do). He was at once bodyguard and servant (он был одновременно телохранителем и слугой; at once — одновременно). But I forgot that being partly a man (но я забыл, что будучи частично человеком; to forget — забывать), he could not be submerged into a mere shadow of myself, like a true animal (он не мог бы погрузиться лишь в мою тень = стать лишь моей тенью, как истинное животное; to submerge — окунать; погружать). Apparently his semi-brain retained impressions of hate, resentment, and some sort of bestial ambition of its own (очевидно, его полу-мозг хранил впечатления/ощущения ненависти, чувство обиды и какое-то собственное звериное честолюбие; to retain — держать; сохранять; помнить; impression — впечатление; ощущение;resentment — негодование, возмущение; чувство обиды).