Hi.
Item 1 reads OK, but Item 2 has me concerned about: "The main flaw in his essay is the sentence structures" i.e. combining singular with plural.
I'm not a teacher, but, anyone actually using such comments should be very cogniscent of their effect on the student.
We are in the process of doing annual appraisals at the moment and one of the things that we emphasise is that it is better to be positive, rather than critical when reviewing people's work.
On a "Writing Job Descriptions" course, recently, we were advised that it can cost over 7 times an employee's annual salary to replace them - and there's a currently a major shortage of 30-40+ year olds available in our industry to take the place of those planning on retiring soon. We therefore really have to consider training before criticism.
In your examples, you may wish to suggest something such as: "..... could benefit significantly from extra [detailed] training and practice in sentence structure".
It's a subtle change, but it could make all the difference to whether the student looks ahead in a positive manner or is just totally "turned off" by a negative comment.
Saying that the work, on which someone has possibly spent a lot of time & effort, is "full of flaws" could totally demoralise a student and turn them of their studies.
What do you think?