Hi, Thomes 減肥 is a verb. Describing verb needs adverb. In Chinese every words behind 地, that is an adverb. Such as 快樂地跳著 ( Dancing happily ), 憤怒地看著它( starring at it angrily ) And so on, words behind 的 is an adjective. Like, 快樂的心情 ( happy mind ), 無聊的歌曲 ( boring song) follow an adjective is a noun. according to your sentence, that means you are on diet, but you don't want to be known. you have to do it secretly. so 偷偷的減肥 is not a correct sentence. 偷偷地減肥 is 100% right ! Even so, we still can understand what are you talking about, just some problem on writing! hope you learn Chinese happily ( 快樂地學中文 )

you should offer more information.

秘密的 减肥 = mì-mì-de jiǎn-féi = confidentially lost weight

秘密的减肥？ Guess you've heard the song All About That Bass, so that is "My momma she told me don't worry about your size~~~but she tries to lose much weight herself when I'm not beside~~~Em booty booty~ Em booty booty~ "

偷偷地减肥？