How he knew that the time had now come when sap should stir in the trees and life begin to move in the soil he could not have told himself.

这个句子，原本就是个陈述句。

How he knew that the time had now come when sap should stir in the trees and life begin to move in the soil， 是told 的宾语从句。这叫“宾语前置”，为了达到强调宾语的目的，此外，也是为了句子匀称的缘故。

我把语序改为正常：

He could not have told himself how he knew that the time had now come when sap should stir in the trees and life begin to move in the soil. 

1. could not have told：是对过去情况的推测，意为“不可能说出；讲不明白（某种道理）”。

2. himself 在这里作he 的同位语。意为：自己说出...

3. 在knew 的宾语从句中，又包含着一个定语从句，修饰the time，而且先行词和定语从句被谓语动词had now come 分隔。

4. begin 的前面，是承前省略了should。

【直译】他自己不可能说出，他如何会知道树里面的汁液开始流动、土壤里的生命开始萌动的时候已经来临。