VOA慢速英语:Farmers in East Africa Struggle Against Cassava Disease

as needed需要的时候。这里的as相当于when，引导时间状语从句。这里是as it is needed的省略形式。

When引导的状语从句中的主语与主句中的主语一致，且其谓语中有be时，可以将主语和be一同省略。如主句和从句的主语虽不一致，但从句的

主语为it且谓语中有be,也可以将it和be省略。该规则也适用于if/though/although引导的从句。

2.He is working to develop disease-resistant plants, but he says it will probably take five years.

probably,Possibly,perhaps,maybe的区别：

这四个副词都表示“或许、大概”的意思。

1）Perhaps和Possibly大体同义，它表示的意思是“也许如此，也许不如此”。例如：

Perhaps the letter will come today.

It is possibly true.

2)Probably的肯定店气最强，有most likely的意味，表示某事可能发生。例如：

The singer will probably succeed.

John will perhaps succeed.

3)maybe比perhaps在用法上较为普通，而又是不很庄重的用语，多用于美国口语，例如：

I think maybe they don't want him,he's so cross.

Maybe it is true.

3.But farmers sometimes do not find the disease until they cut open a cassava.

not until直到……才

not until 引起的时间状语置于句首时，句子的主谓应部分倒装。由 not until 引导的时间状语从句位于句首时，主句应部分倒装，从句语序不变。如：

　　Not until 1998 did he return to his hometown.

　　直到 1998 年他才回到家乡。

　　Not until Father came back did we begin to have supper last night.

　　昨晚直到父亲回来，我们才开始吃晚饭。

　　注意：当 not until 引导的状语或状语从句用于强调句型时，句子的主语不倒装。如将上两句改为强调句应为：

　　It was not until 1998 that he returned to his hometown.

　　It was not until Father came back that we began to have supper last night.