How I regretted the hours wasted in the woods and fields.
regret是“因……而后悔或遗憾”的意思，像keep一样，regret也是一个高频常用词。regret的基本用法有：
I regretted my mistake. 我为自己所犯的错误感到后悔。（regret后直接加名词）
I regretted very much having wasted the precious time. 我浪费了宝贵的时间，感到非常后悔。（regret后直接加ing形式）
I regretted that I haven't learnt the lesson well. 我因没学好功课而感到后悔。（regret后还可加that从句）
I regret to say that I am unable to help you. 我很抱歉，我爱莫能助。（regret后面加不定式短语）
因此课文中的句子可以变为：
How I regretted having wasted the hours in the woods and fields.
How I regretted that I wasted the hours in the woods and fields.
regret用法总结：后面可直接跟名词、动词ing形式、动词不定式以及that从句。

What would I not have given to be able to say the rules without a mistake!
What would I not give ... / What would I not have given to ... 是一种表示强烈愿望的说法，直译是“为了做……事，我有什么不愿给的呢！”即“我什么代价都愿意付出。”或者说“我多么愿意做……事啊！”
英文中表达愿望还有两种更常用的表达方式：wish和if only。例如：
I wish to do well in the exam. 我希望考试考得好。（表示未来的愿望，wish相当于动词want。）
I wish I knew the answer to your question. 但愿我知道你的问题的答案。（表示现在的愿望，后面的过去时knew不表示过去，而表示不易实现的愿望。本句也可用If only I knew the answer to your question.）
I wish I could have said the rules without a mistake. 但愿当时我能把这些规则一点不错地说出来。（表示过去的愿望）
I wish I were a bird. 但愿我是一只鸟。（过去时were表示不可能实现的愿望）
在以上表达愿望的三种方式中，wish和if only表示一般的愿望；而What would I not give to ... / What would I not have given to ... 则是一种表示强烈愿望的说法。

It almost seemed as if the good man were trying to teach us all he knew at this last lesson.
It seemed as if someone were ...是“看来某人是在……”的意思，过去时were并不表示过去，而是表示一种假设。
as if有“好像、仿佛”的意思。例如：
He looked at me as if I were a stranger. 他看着我，好像我是个陌生人似的。
I feel as if we've never parted. 我感到似乎我们从来没有分开过。
Those village people stared at John as if seeing a foreigner for the first time. 那些村民目不转睛地看着约翰，好像第一次见到外国人。
He stood up as if to leave. 他站了起来，像是要走。
Don't stand there as if rooted to the ground. 别像钉在地上般地站在那儿啊。

He stood leaning against the wall.
against是介词，表示“倚在、紧靠着”的意思。例如：
He rested his bicycle against the wall. 他把自行车靠在墙上。
A sheet of paper was caught against the wire by the wind. 一张纸被风刮在电线上。
He rubbed a match against the box. 他在火柴盒上擦亮火柴。