Привет! Давай разберём каждое предложение и найдём ошибки:
My mum and my dad was playing in the sand with my sister.
- Ошибка в согласовании глагола: "was" нужно изменить на "were".
- Правильно: My mum and my dad were playing in the sand with my sister.
You not have to go if you don't want to.
- Пропущен вспомогательный глагол: "do" перед "not".
- Правильно: You do not have to go if you don't want to.
We mustn't run. The train doesn't go for an hour.
- Здесь всё правильно.
You must to be careful. It's very dangerous.
- Лишняя частица "to" после "must".
- Правильно: You must be careful. It's very dangerous.
I played football when I broke my leg.
- Здесь всё правильно.
Yesterday the sports shop was sell them for only £15.
- Ошибка в форме глагола: "was sell" нужно изменить на "was selling" или просто "sold".
- Правильно: Yesterday the sports shop was selling them for only £15.
Если у тебя остались вопросы, спрашивай!