My teacher marked my sentence below as wrong. She said the right word should have been " disappointing " not " disappointed'. Is this true ?
My sentence : The film was not very good. I was disappointed.
My teacher's sentence : The film was not very good. It was disappointing.
Perhaps your teacher took it that as you are talking about the film in the first of the two sentences - 'the film was not very good' - that it was expected that the second sentence was a continuation of this - 'the film was disappointing'
At the end of her correction, she wrote a comment : " Excellent try" . Is this correct ?
Can we say " an excellent try " ? Yes
But it was more than a 'try' - a native speaker would have said just what you wrote!