About Marking an Essay...
Say your criteria are...
a) - the ability to comprehend and understand the provided stimulus.
b) - to understand the conventions of essay writing.
c) - use of language suitable to audience and purpose.
The essay topic was 'Supply your opinion in a five paragraph formal essay whether you prefered the film or novel version of 'Skellig' - I lost two marks out of a possible twenty as I used a quote in my first paragraph from a blog of the author and my essay appeared as a review because of this.
Do you think this is appropriate to the criteria? Should 10% have been taken away because of this?
(question edited somewhat for grammar)
Again, I think part of your difficulty in understanding grades is that you are thinking that students somehow start out with perfect grades (for existing? or what?) and then have marks taken off. That isn't how grading works. You start out with a zero or F, before you hand in your first assignment because you haven't yet *earned* marks. Then, when you hand in an assignment, and the teacher grades it, you are gradually awarded marks for what you have done, with respect to the criteria of the assignment.
Many things affect your marks, some of which teachers will address in specific comments and some of which will not be mentioned specifically. For example, many English teachers will not award full marks for work with grammatical errors. Given your post above, this may be an issue (e.g the plural of "criterion" is "criteria").
Second, there is a generic difference between a formal essay and a review. Without having the essay in front of me, I can't comment on details, but it sounds as if your teacher thought that you did not fully understand the difference in genre.
Blogs are often not considered acceptable research resources in formal essays.
Overall, your grade does sound appropriate given your description of the criteria.