1. We need to reduce our plastic waste; otherwise, we will threaten our own existence. (If)
-> If we don't reduce our plastic waste, we will threaten our own existence.
2. Attend the class regularly, and you can pass the test easily. (I)
-> If you attend the class regularly, you can pass the test easily.
3. Don't drink that water, or you become ill. (If)

-> if you don't drink that water, you will become ill. 
4. We take action now, or the deaths of aquatic plants and animals will increase. (If)
-> if we take action now, the deaths of aquatic plants and animals will not increase.
5. People get more diseases because the water is contaminated. (If)
6. Fishermen use much dynamite to catch fish, so the water is pollute
hd. (If)
7. Emergency workers took flood victims to safe areas last night. (passive voice)