A. Positive reinforcement is the only kind that has any effect. It's much harder to build on weaknesses. The students have heard so much that's negative from teachers in the past. If strengths are praised, weaknesses wither away, once students develop confidence. For example, I don't teach grammar.
Nobody speaks in sentence fragments.
The most I'll do with sentence errors is read the material aloud to students. Their ears are perfect. They lack confidence, so they invent an artificial, awkward voice. They should respect their own voices. My response to dialect, for example, is to suggest that students show their work to someone, and that they speak to professors, explaining the situation clearly.