I am having a big trouble about answering questions like "Do you like swimming ?". Though, this is a very simple language, I've always thought that it is correct to answer by saying "Yes, I like swimming because..." or "No, I don't like it because...". But today, an IELTS tutor strictly underlined that I will lose mark, he said that it is grammatically incorrect to start with "Yes, I like..." Please note that the tutor doesn't claim it is not just inappropriate for IELTS, it is incorrect in general he says. I asked a native, who is also a teacher, there is a controversion as the second teacher said that it is grammaticaly correct to answer by saying "Yes, I like..." Please answer if it is grammaticaly correct or incorrect to answer a "Do you like sth/doing sth" question by beginning with "Yes, I like" in informal speaking. I'm not asking specifically for IELTS. What is the accepted view for it, what is the rule, is it really grammaticaly incorrect ? Thank you.
Your tutor could argue that the usage is a comma splice, where "yes/no" is the answer to the question and "I like/dislike..." elaborates on this answer in a separate clause which should be separated from the first word by a semicolon.
Yes; I like swimming because...
But I'd be surprised to see any native speaker use a semicolon in this scenario. The use of a comma here is idiomatic and in my view your tutor is wrong to deduct marks for it.