1.secret作名词时的意思是“秘密,机密”,也可作“奥秘”“秘诀,诀窍”解,此时只用作单数形式。
2.secret还可作“奥秘,神秘的事物”解,此时用复数形式。

in secret
偷偷地,暗地里 without others knowing
They met in secret to discuss the arrangements.他们秘密会见,商议了各种安排。
He had left the country in secret.他已秘密出国。
He married her in secret.他秘密地娶了她。

in the secret
知道内情 among those who know the secret
Is Phil in the secret?菲尔是不是也知道其中的内情?

let in on a secret
让某人知道秘密 let sb know the secret
▲let sb in on a secret
They talked in a suppressed voice as he were letting her in on a secret.他们谈话时声音压得很低,好像他在告诉她一个秘密似的。
If I were you,I wouldn't let her in on the secret.如果我是你的话,我不会叫她知道这个秘密。

She spilled the secret.她泄露了机密。
They told me not to reveal the secret.他们要我不要泄露机密。
He was on his honour not to tell the secret.他以名誉担保不把这个秘密告诉别人。
Guard the secret with your life, tell it to no one!用生命保守住这秘密,别告诉任何人。
I couldn't resist telling him the secret.我忍不住要把秘密告诉他。
His reputation had decided me to confide a secret to him.他的声誉使我决定向他吐露一桩秘密。
The wedding date is a big secret.举行婚礼的日期是一大秘密。
He made no secret of his dislike for me.他并不讳言他不喜欢我。
What's your secret for this wonderful pastry?你做的酥馅饼真好,有什么诀窍?
The secret to more effective writing is simple:talk to your reader.把文章写得更有效果的诀窍很简单,那就是同读者谈话。
The scientists are searching after the secret of life on earth.科学家们正在探索地球上生命的奥秘。
▲～+of v-ing
What's the secret of baking perfect bread?烤出上好面包的秘诀是什么?

in secret, in the secret
这两个短语在意思上和用法上都不同：前者作“秘密地”或“偷偷地”解,作状语,相当于secretly; 后者则作“知道秘密”解,多用作表语。

不要把这件事告诉别人。
［误］  This is to be kept a secret to the others.
［正］  This is to be kept a secret from the others.
［析］  secret如用作名词,可用keep sth a secret from或keep a/the secret from的结构表示“对…保守秘密”。

［派生词］ secretive adj.遮遮掩掩的,守口如瓶的,秘密的‖secretively adv.隐秘地
［同义词］ adj.  backdoor, close, deep, concealed, undercover, underhand, unpublished, unseen, covert, confidential, classified n.  confidence, mystery, tips
［反义词］ adj.  open, public, overt
［词源］  ＜拉丁语secretus(隐藏起来的)

1.secret的基本意思是“秘密的”,指躲藏、躲避或被隐藏起来,不让别人注意到或看到,在句中可用作定语或表语。也可作“保密的或不公开的”解,在句中作定语。
2.secret无比较级和最高级形式。

用作定语
▲～+n.
He was tortured by the secret police, but they could not break his spirit.他遭到秘密警察的拷打,但他们并不能摧毁他的精神。
They were forced to publish the secret news.他们被迫公开秘闻。
You can't fool me,I know about your secret plan!你骗不了我,我知道你的秘密计划!
He kept his money stored away in a secret place, lest his nephew should lay his hands on it.他把钱藏在一个隐秘处,提防他侄儿拿到。
That is my secret cottage in the country.那是我在郊野的秘密住所。
The kind medicine is based on a secret formula.这种药是根据一种秘方制成的。
He's a devious, secret person and I don't trust him.他是一个狡猾诡秘的人,我不信任他。
We discovered a secret passage behind the wall.我们在墙后发现了秘密通道。
John is a secret admirer of Helen, though he has never spoken to her.约翰暗自爱慕海伦,尽管他从未对她说过。
I'm a secret fan of soap operas on TV.我从不声张我是电视剧迷。
用作表语
▲S+be+～
 The plan is secret.这计划是秘密的。
These plans must be kept secret from enemy.这些计划一定要对敌人保密。
▲S+be+～+prep.-phrase
 He is rather secret about his private affairs.他对自己的私事有点守口如瓶。
用作宾语补足语
▲S+V+O+～
He kept his illness secret from his wife.他不把自己的病告诉给他的妻子。

请对我们结婚的事尽量保密。
［误］  Please keep our marriage in secret as far as possible.
［正］  Please keep our marriage secret as far as possible.
［正］  Our marriage was kept in secret as far as possible.
［析］  “对某事保守秘密”用keep sth secret的结构, secret在此是形容词,作宾语补足语。in secret(秘密地,暗地里)一般只能作状语,可以用sth to be done in secret的形式。