The air must be kept clean.
Water must be used efficiently.
The garbage must be put in special containers.
Cars should not be parked near houses.
Trees must not be cut in big numbers in forests.
И без модальных глаголов
The air is kept clean and we can breathe.
The garbage is put in special containers and we can remain healthy.
Cars are not parked near our houses and we can have cleaner air.
Trees are not cut in big numbers and we have enough air to breathe,
Water is used efficiently and we have enough water to drink.