偷偷 emphasizes that an action is done with the intention that other people won't notice it.
In a sentence, it usually serves as an attribute and is placed before a predicate.
Pay attention
偷偷 is often followed by the particle 地 .
Examples
我偷偷地寄给她一束花
I secretly sent her a bouquet of flowers
wǒ tōutōu de jì gěi tā yí shù huā
我知道有人偷偷看了一眼我的日记
I know that someone took a peek at my diary
wǒ zhīdao yǒu rén tōutōu kàn le yí yǎn wǒ de rìjì
她以为没人发现她偷偷地拿了一块蛋糕
She thought no one had noticed her sneaking a piece of cake
tā yǐwéi méi rén fāxiàn tā tōutōu de ná le yí kuài dàngāo
The adverb 悄悄
The adverb 悄悄 emphasizes that the action is done without noise.
In a sentence, it usually serves as an attribute and is placed before a predicate.
Pay attention
悄悄 is often followed by the particle 地.
Examples
兔子悄悄溜走了
The rabbit slipped away
tùzǐ qiāoqiāo liū zǒu le
因为迟到了,我悄悄地走进教室,坐在最后边
I quietly entered the classroom and sat in the last row because I was late
yīnwèi chídào le, wǒ qiāoqiāo de zǒujìn jiàoshì, zuò zài zuì hòubian
悄悄 as an adjective
悄悄 can serve as an adjective and have the meaning "quiet". 悄悄 is usually preceded by the adjective 静 to intensify its meaning.