He forgot to turn the tap off.

他忘了关水龙头。

You must always turn the tap off.

你应该把水龙头关掉。

Turn the tap off. Don't let it waste.

把水龙头关上，别让水白白流掉。

Turn the tap off, when you have down.

洗完手后，把水龙头关了。

Turn the tap off, when you have done.

洗完手后，把水龙头关了。