Students don't have poor comprehension because they lack knowledge of grammar. Learning disabilities also throw a curve in the learning process. Teaching to the student's individual learning style, providing as much practice with as many different strategies will be more successful than sticking our heads in the sand and sticking to only one approach. I have experienced great success with colouring different parts of speech ie red nouns, purple pronouns, orange adjectives, black verbs etc.
All English courses reinforce the same colour code.
Even prepositions are making sense to some who could never grasp these concepts. I don't know how to attach Peter's letter.