We can know from the book the air is very fresh 这句话有没有问题

没有的,可以在book后加上that,但这个是可以省略的。from the book 的位置可以放句首或者句末。句子意思不变：我们可以从书中得知空气是很新鲜的。 We can know from the book that in the past the air there was very fresh。 这里,加了that,in the past 表示“在过去”,there 是那儿的意思,最后这个意思你没有要求。 如果,你在上文中已经讲述了具体的过去时间,句子最好说成：We can know from the book (that) the air then was very fresh。