You can use both and both are correct. However, I feel there's a very slight difference between the two. You learn English seems more like you have been learning English (for years) and will continue learning it (as a literature subject or something) for years to come. But I guess you are just learning this at the moment as a process to improve and you'll stop learning it once you reach your goal. In this continuous sounds more appropriate. Same with aerodynamics. However, if you say I teach English, it implies teaching is your profession and it is going to be your profession for your entire life. I am teaching English implies I am teaching English today. Next year, I might teach something else.