Don't go on a diet.

不要节食。

I'm supposed to go on a diet.

我应该节食。

Helen: I need to go on a diet.

海伦：我需要节食减肥。

Your rabbit should go on a diet!

你的兔子该减肥了！

Mary's supposed to go on a diet.

汤姆和玛丽应该节食。