Хотя я не на диете, я тоже стараюсь есть меньше калорий вечером.

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Breakdown of Хотя я не на диете, я тоже стараюсь есть меньше калорий вечером.

• есть: to eat
• на: on
• не: not

Questions & Answers about Хотя я не на диете, я тоже стараюсь есть меньше калорий вечером.

Why is it я не на диете and not something like я не диета or я не на диета?

In Russian, быть на диете (literally "to be on a diet") is the standard idiom for "to be dieting."

• диета is a noun, feminine, nominative singular.
• After the preposition на in the meaning "on / at (a state or regimen)," Russian uses the prepositional case: на диете.
  
  • на работе – at work
  • на пенсии – retired (on a pension)
  • на диете – on a diet

You cannot say я не диета because that would literally mean "I am not a diet" (no preposition, wrong meaning).

You also can’t say на диета because на requires the noun to change case: диета → диете in the prepositional.

So the correct form for "I’m not on a diet" is я не на диете.

What exactly does Хотя mean here, and could the clauses be reversed?