Must and mustn't are used to express obligations and prohibitions in English. Must is used to show something that is required or necessary, similar to the Turkish suffix "-meli, -malı". Some examples given are that we must give our seat to the elderly on public buses, help the disabled, brush our teeth, do homework, and come to school on time. Mustn't is used to show things we shouldn't do, such as eating or drinking on public buses, disturbing others, making noise in class, or not obeying school rules.