1) Mum says that I shouldn't skip my breakfast because it can cause a stomachache
2) Dad says that I shouldn't drink beer at my age because it can cause obesity
3) Mum said that I should follow the diet because it can do me well
4) The doctor asked if I have health health problems
5) Doctor says that I need to eat high-fibre food more often because it will make me feel good
В первых двух предложениях "shouldn't" можно заменить на "mustn't". Отталкивайся от своей школьной программы, потому что могут снизить оценку даже за такую мелочь, хотя смысл не теряется.