The conventional theory is that people yawn when bored or sleepy because yawning raises blood oxygen levels, which in turn raises alertness.whchi in turn 在这里不是否定吧？是进一步吗？
It’s worth remembering, however, that though a clean copy fresh off a printer may look terrible, it will read only as well as the thinking and writing that have gone into it.问一，粗体部分的语法结构。问二，为什么两个分句之间没有连词？
As a result, she says, some of the injuries associated with
running, such as runner’s knee, are uncommon among race walkers.请问这里的 are 是与哪个主语保持一致?这样的问题如何定位主谓一致?
And last year Khan Academy and the College Board unveiled a free course they say boosts SAT scores for students at all income levels.这里的黑体部分是插入语吗，如果是为什么没有逗号呢？如果不是那是什么成分?
This is the question about which we've had so much discussion这句话中为什么要有个介词about？Everything that he said seemed quite reasonable这句话中为什么又没有介词了？介词什么时候，用什么时候不用？？