1. We can turn off the water when we are not using it.

Question

1-1. We can turn off the water when we do not use it.

2. We can turn off the water when we are brushing teeth.

2-1. We can turn off the water when we do not brush teeth.
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Which ones are grammatical? Do we have to use the ing form?

Answer 1

1 and 2 are correct. The gerund form is useful here. 1-1 is awkward. 2-1 makes no sense.