1. We can save environment when we don't make junk any more.

2. If you run every day, you will get health.

3. We can save the electricity when we turn off the computer.

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Q1: Are they all grammatical?
Q2: Do we have to use 'the' before electricity or not?

Corrected:

1. We can save the environment when we don't make junk any more.

2. If you run every day, you will be healthier than if you sit around all day.

3. We can save electricity when we turn off the computer. [no "the" in front of "electricity"]