Question

1. We can turn off the tap of water, when we are washing our faces.

2. We can turn off the tap water, when we are washing our faces.

3. We can turn off the city water, when we are washing our faces.
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Q1: Can we use all the expressions?
Q2: Can we use 'wash a face' instead of 'washing our faces'?

Answer 1

#2 is correct if you remove the comma.

#3 is OK, but #2 is better.

#1 is not correct.

No, "wash a face" will not work, but if you're looking for a singular, here's this:

I can turn off the tap water when I'm washing my face.

You ...

He ...

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