1. 这里应该理解为：到退休前的那一天，他将从教整40年。（on) the day before his retirement. 主句使用将来完成时。如果改用by从语法角度没错，但未必完全符合原意，因为by = on/before，不一定就是那天，也可能早一两天。请看例句：1. We will have been married a year on June 25th.（《朗文英语语法》例句） 2. On 14 October I will have worked here ten years.《剑桥词典》https://dictionary.cambridge.org/zhs/%E8%AF%8D%E5%85%B8/%E8%8B%B1%E8%AF%AD/future-perfect

2. 原文没有问题，当然你如果改用came away from his hands也对。也可以说came out of his hands.其实几个意思都差不多。你要不断熟悉英语不同形式的表达。请看外刊例句：

1. His wooden cup came away from his hands and landed at the feet of the Lord. 

2. Just as the forked spear started to come away from his hands, she brought the baton around in a tight semicircle to crack on top of it. 

3. Then the apparatus suddenly came out of his hands and he felt pain on the under side of the left forearm between the hand and elbow.

好问题忍不住说一下。第一个问题“好题”已经说了，这种将来完成时用by的句子本身有一定的模糊性（注意不是歧义的问题），by可以理解为未来的某个时间点，也可理解为未来的某个时间点之前。因此不用by改用某个确定的未来时间点也是可以的：

At the end of the summer，the Blakes will have been married for 10 years.

By the end of the summer，the Blakes will have been married for 10 years.

“the day before his retirement”可以直接作为修饰状语，挺多见的，且加by还显得多余：

A year from now she will have written twelve books.

第二个问题是个很经典的问题。用came away from 的意思是： 似乎the steering wheel和手本来粘在一起，结果脱落了，很奇怪的意思。came away in sb‘s hand是个固定短语（=become separated very easily or without you realising it） ，表达的完全是另一个意思。

I took hold of the steering wheel and it came away in my hand.

The paper has started to come away from the wall because it’s so damp.

I just opened the drawer as usual and the handle came away in my hand.