主语就是than，你可以把than看作带有比较作用的that，这样应该会好理解点

Q1:  谁是它的主语？

  A1:  主语 = treatments (复数), 所以 than ARE (复数)


Q2: 为什么感觉  than are possible 缺主语？

  A2: 直述 = treatments THAT provide more detail than space** allotted for this introduction here would allow = treatments THAT are more detailed because ....

    THAT 后面的字串描述 THAT 前面的主题 ( 通常是,但不总是紧接 THAT 之前的字 ), THAT 起代名词作用, 经常紧跟主题, 实际上可有可无**

    ** space WHICH IS allotted for = space allotted for       space (单数) 配 which (单数)

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不是  are possible,  是 THAN are possible

引用回帖:

5楼: Originally posted by Fantein at 2016-10-06 10:25:39
感谢回答！
大致懂了，是省略的一种吧…
?… a set of further readings which will provide you with more detailed treatments than (treatments/those) are possible in this introduction.
这样写是完整句子？
...

... a set of further readings which will provide you with more detailed treatments than those THAT are possible in this introduction

  NO:  ... more detailed treatments than treatments   (忌讳重复)

YES:  ... more detailed treatments than those  ( 第二次提 "treatments"   用 "those" )    <=  treatments / those (复数), 所以用 that 不用 which (单数)
，

引用回帖:

7楼: Originally posted by wang8102 at 2016-10-06 15:17:02
that 做主语还能省略? than的词性有两种，一种是介词一种是连词(可连接两个名词，或者从属连词链接两个句子)，明显不是介词，作为连词链接句子后面缺少主语，而主语在句子里是不能省略的，所以个人认为是句子吧tha ...

应该不可能是写错了，这句话出自剑桥大学出版社原版教材。