The question was to transform this sentence from positive to comparative degree: She is not so young as I expected. Following the solutions of other sentences of this kind (the photo shows a similar kind of problem from where I adopted the solution) it can be written that: She is not younger than my expectation. I want to know why ...I expected has been changed to ...my expectation. What is the grammatical rule here? I don't get it what's wrong with ...I expected ? Is it wrong not to use possessive pronoun?
The sentence transformation is from the past exam question of my student which she couldn't solve. I am her private tutor and want to teach her the correct things. Please don't misunderstand it as a homework.