Thank you for your answer. So, "have a seat" and "sit down" are same meaning.
How about using "can" instead of "may"? Do I always need to use "may" when I'm asking someone's permission? For example... "Can I use your pen?" or "Can I come with you?" are not correct? Does "can" always mention possibility? For example... "I can swim for long distance." or "She can touch s