Bahahaha, that's one very solid something really depending on it.🤣 'Si' means something for yourself by yourself I think, like sing or earn money or go for a walk, buy a t-shirt..it turns the narration inside the person. 'Se' is for everything else, but it's used very widely and changes the verb, that one is really nasty for a foreigner.🤔 Just like past tense with 'jsem', especially russians I knew were bleeding on that one.