"I am on a diet."
Translation:Io sono a dieta.
Hate it when DL throws you an unlearnt idiom, no possible way to know it when going from English to Italian. Have reported this.
Why the preposition "a" doesn't change because of the article "la"? Why instead of "a" there isn't "alla" ?
It is idiomatic, as mattiapascal suggests. Also in Spanish we would say: "Estoy A dieta", and it is very rare to use "a" after the verb "ser" (essere).
I got it wrong too - have never encountered this sentence before and it popped up at the end of my 'spiritual' lesson for some reason?!