by now表示的意思为到目前为止，但是后面没有要加消极信息的那一层意思,也没有严格的时间点或地点的要求；so far意为如此程度、迄今为止，表示时间点或地点，后面的句子一般用完成时。

by now例句

She'll have had the results by now.

她现在应该知道结果了。

She would be in bed by now, as like as not.

这工夫她很可能睡了。

If you think about it, those children must be at school by now.

想想吧，那些孩子现在一定在学校了。

He'll be home by now.

他现在该到家了。

If he started out at nine, he ought to be here by now.

他如果九点出发，现在应该到这里了

so far例句

How many driving lessons have you had so far?

你到目前为止上过多少节驾驶课了？

So far, the police investigation has drawn a blank.

到目前为止警方的调查毫无结果。

What do you think of the show so far?

到目前为止你觉得这场演出怎么样？

I've only read a little of the book so far.

这本书我才读了一小部分。

That's the truth, in so far as I know it.

就我所知，那是真实情况。