VOA慢速英语:Scientists Document Effects of Market Policy Changes on Three West African Countries

replace是替换,取代的意思。

replace和sbstitute都表示替代的意思，不过用法上有所不同：

substitute的用法是：substitute A for B，意为用A代替B；

replace 的用法是：replace A with B，意为用B代替A。

例句：
1)In manufacturing, cheaper materials are constantly being substituted for the better and more expensive kind.
2.)The government decided to replace coal with gas.

还有一个词组可以表示替代的意思：take the place of sth，替代某物的位置。

2.She said buying the cheaper imported rice worked well until the worldwide food crisis of two thousand eight.

until是“直到……”的意思，在后面的事情发生之前，前面的动作一直持续。

buying the cheaper imported rice是动名词短语做主语。

3.He said this is often seen as a problem for Mali. But it caused them to depend less on imported rice.

see as将……视为，相当于view as,consider as,regard as.

depend on依赖某物

4.Based on their research, the experts suggest that farmers plant a variety of crops and not just depend on rice.

用来表示要求、建议、命令、提议、意愿等的主语从句、宾语从句、表语从句和同位语从句中用虚拟语气，由
should(可省略)+动词原形that从句构成。这类动词有：

ask要求 advise建议 arrange安排 beg请求
command命令 decide决定 demand要求 desire渴望
determine决定 insist坚持 intend打算 maintain坚持主张
move建议，动员 propose提议 object反对 order命令
prefer建议 require 需要 request要求 resolve下决心
recommend推荐 suggest建议 stipulate约定，规定 urge强调，促进
vote公认，提议 decree颁布（法令） pray请求

注意：这类动词后面除了可接that虚拟句以外，也可以接doing或者to do…来表达，一般情况下，意思没有什么差别。
She advised that we should keep the gate locked.（书面体）
She advised us to keep the gate locked.（口语）
She advised keeping the gate locked.(较随便)