Don't use plastic bags to save the environment.

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What is the meaning of 'save' here?
what other words can we use instead of 'save'?

Your sentence is misleading.

It would be better to say,

To save the environment, don't use plastic bags.

or

Don't use plastic bags; they hurt the environment.

or

To conserve the environment, don't use plastic bags.

I would use the synonym of "conserve."

In the given context, the meaning of 'save' is to protect or prevent harm to the environment. It suggests taking necessary actions to minimize the negative impact on nature caused by the use of plastic bags.

There are several other words or phrases that can be used instead of 'save' to convey a similar meaning. Some alternatives include:

1. Preserve: This word emphasizes the idea of maintaining or keeping the environment in its original, clean, and natural state.
2. Conserve: This word highlights the idea of using resources, such as plastic bags, judiciously and avoiding wastefulness.
3. Protect: This word focuses on shielding or safeguarding the environment from harm or damage.
4. Sustain: This word suggests the idea of supporting the environment's long-term vitality and ensuring its continued existence.
5. Reduce: This word specifically refers to minimizing the use of plastic bags, which in turn helps in minimizing the negative effects on the environment.

It's important to note that while these words have slightly different nuances, they all share the common goal of promoting environmental well-being.