1. secretly/in secret/in secrecy: done without anyone else knowing

例：

Many civilians were secretly killed by soldiers. （士兵秘密处决了很多平民。）

My parents didn't approve of our relationship, and we had to meet in secret. （我爸妈不同意我们交往，所以我们只能偷偷见面。）

2. on the quiet: if you do something on the quiet , you do it secretly, especially because you think that people will disapprove of what you are doing

例：

His doctor has told him he mustn't drink, but he still has the occasional brandy on the quiet. （他的医生告诫他不能再喝酒了，但是他偶尔还是会偷偷喝点白兰地。）

He used to flirt with the two girls, on the quiet, when his wife wasn't looking. （他常常趁他老婆不注意跟这两个女孩调情。）

We found out he'd been doing some freelance work on the quiet. （我们发现他暗地里做一些自由撰稿的工作。）

3. behind closed doors: if important official meetings, discussions, or decisions take place behind closed doors , they take place secretly without the public being allowed to see or hear them

例：

The board members met behind closed doors to discuss the deal. （董事会成员们私下见面讨论这桩生意。）

4. in private/privately: if you do something in private or privately , you do it where other people cannot see or hear you

例：

Can I have a word with you in private? （我能私下里和你说句话吗？）

Although party officials give the President their public support, many are saying in private that he may have to resign. （虽然这些官员明里支持总统，但其中很多人背地里都在说总统该下台了。）

Generations of Native American children in state schools were punished for speaking their own language, even privately. （一直以来，上公立学校的美国土著儿童如果在学校讲自己的土话就要被罚，即使偷偷讲，发现了也要罚。）

5. behind somebody's back: if you do something or say something unpleasant about someone behind their back , you do or say it without telling them

例：

I thought you were my friend. Now I find you've been talking about me behind my back. （我原以为你是我的朋友，没想到你在背后说我坏话。）

People laughed at him behind his back. （大家在他背后偷偷笑话他。）

He agrees with his boss to his face, but then criticizes him behind his back. （他当着老板的面应声附和，背后却批评老板。）

go behind somebody's back

I'm not happy about you going behind my back like that. You should have told me. （我不喜欢你在背后搞小动作，你应该当面跟我说。）

6. surreptitiously: if you do something surreptitiously, you do it while other people are not looking because you do not want them to see you doing it

例：

She glanced surreptitiously up at the clock. （他偷偷看了看钟。）

I found myself studying his face surreptitiously. （我不由自主地一直偷偷看他。）

Greenpeace claims that toxic waste has been dumped surreptitiously on west coast beaches. （绿色和平组织称有毒废料被偷偷倾泻在西海岸。）

7. furtively: if you do something furtively , you do it in a way which makes you look as if you are keeping something secret, especially something wrong that you have done

例：

She looked around furtively to make sure no one was watching. （她偷偷四处张望了一下，确定没人在看。）

She opened the door and looked furtively down the hall. （她打开门，偷偷向厅里看了看。）