To Su.Ki and Denise I may show you an very interesting and common mistake many of Chinese ever made. I seem to remember I posted on italki in the past. When I asked my fellows: "How are you doing" or "How are you going", more than 50% people replied : "I am doing homework" or "I am working" and so on. Around 30% of people did not reply anything I guess they did not ascertain what I was asking. Only 20% of my fellows replied in the proper way. How are you doing How are you going There 2 phrases are most mistakes that puzzled my fellows so far as I know

There isn't just one common grammar mistake - there are many. Did you mean to ask 'What is the MOST common grammar mistake in English?' If so, the commonest mistake among learners is missing the 's' off the the end of the verb for he, she and it in the present simple : for example 'He speak' instead of 'He speaks'. The category of mistake you see here on italki is a total ignorance/total disregard of the rules for asking a simple question.