To be brutally honest, you've made a couple of mistakes here and there, but nothing major. If this were an English paper, you'd probably lose a few marks, but you'd still pass the exam, so I wouldn't worry too much about it. People who've been taught to read and write English for 12 years at school make more (and worse) mistakes than you do. You get your point across and give reasoned arguments, which are far more important at the end of the day.