Fill in the blanks with the correct modal
You _________leave your house unlocked when you go out.

shouldn't

The correct modal to fill in the blank is shouldn't. The modal shouldn't is used to express advice or a suggestion, and it is the most appropriate modal to use in this context because it conveys the idea that it is not a good idea to leave your house unlocked when you go out. Leaving your house unlocked could make it easy for someone to break in and steal your belongings.

The other options are incorrect. The modal couldn't is used to express past ability, but it does not convey the same sense of advice as shouldn't. The modal have to is used to express obligation, but it is too strong in this context. The modal must is also used to express obligation, but it is even stronger than have to.