重点讲解：
1. for free
义务地；无偿地；免费地；
eg. You can hardly expect her to do it for free.
你不可能指望她无偿地做这事。
eg. This paper cannot be downloaded for free.
这篇文章不可免费下载。
2. keep down
使（数目、大小、数量等）保持在低水平；抑制…增长；
eg. She is managing to keep down her spending.
她在设法缩小开支。
eg. "Keep your voice down," I whispered.
我轻声说：“小声点儿。”
3. keep a lid on sth.
限制；抑制；隐瞒；
eg. The soldiers' presence seemed to keep a lid on the violence.
士兵的存在似乎抑制住了暴力活动。
eg. They tell us to keep a lid on the report until the campaign is over.
他们告诉我们竞选结束之前，先隐瞒那份报告。
4. keep quiet about sth.
将…保密；保守…的秘密；
eg. I told her to keep quiet about it.
我告诉她此事要保密。
eg. I've decided to resign but I'd prefer you to keep quiet about it.
我已决定辞职，你先别声张。
