The difference is actually much more than that. As you know, in Korea they have their own way of counting age that makes people a year or two older depending on their date of birth and the time of the year. They use 살 for this one. Now when it comes to legal age, the one that we are used to using everywhere else in the world, they use 세. So when it comes to laws, they use 세. the drinking age is 19세, 19 international age. 19살 could include 17 or 18 year olds in international age, 세 age.

To be honest, I hear the former expression more much than the latter. -- The president enacted many tough on crime policies. -- He was just trying to help you. You didn't have to be so tough on him. The second one makes me think more of a specific situation. "Get tough with (someone)". For example, a child is arguing with her mother and the mother replies by saying "Don't get tough with me". That is my opinion as an American English speaker. There might be regional differences that I may not be aware about (:

it sounds fine! you could also say... 죄송하지만 한국어를 잘 못해요 or 아직 멀었어요 the second expression is a bit more advanced but it means something similar to "I still have a long way to go". So it can be used for anything that your learning, not just Korean (:

체류하다 is more similar to "reside" is in English. You'll see this word a lot in things related to immigration. 체류자격변경 - Changing residential status (visa type) 한국에 체류하는 외국인 - Foreigners residing in Korea 머무르다 is "to stay", much more general as explained in the other answer

It would be more helpful if you posted the full context of whatever you want translated instead of just fragments. They're out of context so we have no idea about the situation those words were used in.

It would be more helpful if you posted the full context of whatever you want translated instead of just fragments. They're out of context so we have no idea about the situation those words were used in.

You need to use 무엇 when you are taking the TOPIK or doing any sort of formal work (reports, university assignments, etc) 뭐 is fine for Kakaotalk, messaging, etc

the only way to really increase fluency is by speaking speaking and speaking!