Grammar in a Nutshell CD 2 Track 6
Everything You Ever Wanted to Know About Grammar... But Were Afraid to Use
English is a chronological language. We just love to know when something happened, and this is indicated by the range and depth of our verb tenses.
I had already seen it by the time she brought it in.
As you probably learned in your grammar studies, "the past perfect is an action in the past that occurred before a separate action in the past." Whew! Not all languages do this. For example, Japanese is fairly casual about when things happened, but being a hierarchical language, it is very important to know what relationship the two people involved had. A high-level person with a low-level one, two peers, a man and a woman, all these things show up in Japanese grammar. Grammatically speaking, English is democratic. The confusing part is that in English the verb tenses are very important, but instead of putting them up on the peaks of a sentence, we throw them all deep down in the valleys! Therefore, two sentences with strong intonation—such as, "Dogs eat bones" and "The dogs'll've eaten the bones" sound amazingly similar. Why? Because it takes the same amount of time to say both sentences since they have the same number of stresses. The three original words and the rhythm stay the same in these sentences, but the meaning changes as you add more stressed words. Articles and verb tense changes are usually not stressed.
Now let's see how this works in the exercises that follow.内容来自 听力课堂网：http://www.tingclass.net/show-8155-232284-1.html