用英语解释这几句句子He kept watching TV until his parents came back Yo

用英语解释这几句句子
He kept watching TV until his parents came back
You shouldn't pollute the water
What do you mean by liquid gold?
The mountain is covered with young trees
第一句没有关键词,第二句的关键词是pollute,第三句也没有关键词,第四句关键词是covered with（关键词是下面划横线的单词,我不会打横线- ）记住,是用英语解释句子!而不是中文翻译,我怕有的仁兄不懂.