求助：看knewton 语法的时候遇到的几个问题

hsy93615

Which can not refer to abstract idea从句不能指代抽象概念吗？
Taylor broke her leg, which kept her from competing in the race. 我感觉这句话没问题啊？那这句话正确说法是什么样的呢？


Summative modifiers
Begin with a or an and a noun that renames or summarizes the gist of the previous clause. Summative modifiers often fix other modifier errors because they don’t need to describe a specific noun.
Eg: The researchers hypothesized that the old method is flawed, which is (a proposal that is) likely to cause a great deal of controversy. 为什么只有括号里面的表达可以，高中学的which非限制性定语从句不是可以修饰前面整个句子，这块我记不太清楚了?


Like and unlike must compared with two nouns.?? 连接ing分词的不行吗？我只记得要遵守严格平行？一定只能连接名词吗?


谁能给我解释一下And once … and… 举几个例句呗？？




我在看knewton 语法的时候遇到的几个问题，请好心人帮我解答一下谢谢啦~