1) I ..... be really mean to her, but i don't want to treat her like that 2) You .... Follow the law or yoy will go to

1) I could be really mean to her, but I don\'t want to treat her like that.

2) You must follow the law or you will go to jail.

3) I can`t go to the wedding. I have to work on the same day.

Пояснение: в пропуски нужно было вставить модальные глаголы could, must и can в зависимости от контекста (перевода) предложений.