Towns and cities were established not only as garrisons (military posts) but as centers for the diffusion of Greek language, literature, and thought, particularly through libraries, as at Antioch (in modern Turkey) and the most famous of all,at Alexandria in Egypt, which would be the finest in the world for the next thousand years. (TPO43, 55)

garrison /'gærəsn/ n. 卫戍部队，守备部队

diffusion /dɪ'fjʊʒən/ n. 散布，普及

大家自己先读，不回读，看一遍是否能理解

Towns and cities were established not only (as garrisons) (military posts) but (as centers for the diffusion of Greek language, literature, and thought), (particularly through libraries), (as at Antioch (in modern Turkey) and the most famous of all, at Alexandria in Egypt), (which would be the finest in the world for the next thousand years). (TPO43, 55)

托福阅读长难句分析：

修饰一：(as garrisons) ，介词短语

中文：作为驻防之地

修饰二：(as centers for the diffusion of Greek language, literature, and thought) ，介词短语

中文：作为传播希腊语言、文学、思想的中心

修饰三：(particularly through libraries) ，插入语

中文：尤其通过图书馆

修饰四：(as at Antioch (in modern Turkey)and the most famous of all, at Alexandria in Egypt) ，介词短语

这里的as是一个副词，表示“像，如同”

中文：就好像在Antioch(现在的土耳其)的图书馆以及最著名的在埃及亚历山大的图书馆

修饰五：(which would be the finest in the world for the next thousand years) ，从句

中文：在接下来的一千年里都是世界最好的。

托福阅读长难句参考翻译：

城镇和城市建立起来，不仅作为驻防之地(军事驻地)，也是传播希腊语言、文学、思想的中心，尤其通过图书馆传播，就好像在Antioch(现在的土耳其)的图书馆以及最著名的在埃及亚历山大的图书馆，该图书馆在接下来的一千年里都是世界最好的。

这个句子的主要修饰成分就是从句、介词短语和插入语，大家务必想清楚，每一个修饰成分修饰的是什么，才能看清楚这个句子的结构。