For some he was the leading figure of the Modernist movement who redefined what poetry was and could be; and who, in his role as cultural advocate, gave vital impetus to the literary careers of T.S. Eliot, James Joyce and Wyndham Lewis, among others.
这是一个复合句。主干部分是he was the leading figure of the Modernist movement，两个who引导的定语从句都是修饰表语the leading figure的；后一个定语从句中，in …介词短语是从句的状语，也可以看成是插入语。
leading figure 领导人
give vital impetus to 给予巨大的推进、促进
对于一些人来说，他是现代主义运动的领导人，他重新定义了什么是诗歌及诗歌应该如何等问题。而作为文化倡导者，他大力推进了T.S. Eliot、James Joyce和Wyndham Lewis等人的文学事业。
Judith Herrin, a professor at King's College London, sets out to show that there are far better reasons to study and admire the civilization that flourished for more than a millennium before the conquest of Constantinople in 1453, and whose legacy is still discernible all over south-east Europe and the Levant.