1. It was missing a piece.

(It refers to 'the circle' One piece was felling from the circle. In this situation, what is the meaning of Sentence 1? Was the piece in the process of falling away from the circle?)

2. You should not make a fire in the mountains.

3. You should not pitch a tent near a river. Especially, when it is rainy, it's dangerous to pitch a tent near a river.

4. You should keep the safty rules.

5. You should buy firewood from a store and take it to the camp.

6. You should not leave food on the table outside, for animals may eat it.

7. You should be careful when you chop the firewood. It may hurt you.

8. You should not build a fire near a tent.

9. You should not wear shorts, for a snake might bite your leg.

10. You should not run when there are lots of branches, for you may hurt your eyes.

11. When you make a tent with tools, you should be very careful.

12. You should not let children swim in the river by themselves.

(Are the expressions all right? Would youi check them? Thank you.)

1 is correct -- it simply means that a piece is not there. It could be falling at the moment or it could have fallen out or been missing for quite a while. Think of a pizza:

(Broken Link Removed)
Once that piece is gone that the person is lifting out, then you could say, "It was missing a piece.

2 - good

3 - The second sentence either needs to be reworded or connected to the first sentence. As it is, the second reads like a fragment, not a complete sentence. Try this:
You should not pitch a tent near a river. When it is rainy, it's especially dangerous to pitch a tent near a river.

4 - good; just correct the spelling of "safety."

5 and 6 - good

7 - good; you need to say what "it" is, though. Do you mean the ax might hurt you?

All the rest look really good.

1. The meaning of sentence 1 is that the circle was missing one piece, which implies that the piece was no longer attached to the circle. This could mean that the piece was in the process of falling away from the circle or it had already fallen off.

Regarding the other expressions, they could be improved for clarity and grammar. Here are the revised versions:

2. It is not advisable to start a fire in the mountains.

3. You should avoid pitching a tent near a river, especially during periods of rainfall as it can be dangerous.

4. You should follow safety rules.

5. You should purchase firewood from a store and bring it to the camp.

6. Do not leave food on the table outside, as animals may eat it.

7. Exercise caution when chopping firewood, as there is a risk of injury.

8. It is important not to build a fire near a tent.

9. Avoid wearing shorts, as there is a possibility of getting bitten by a snake.

10. Do not run in areas with many branches, as they may cause injury to your eyes.

11. When using tools to set up a tent, be extremely careful.

12. Do not allow children to swim in the river unsupervised.

These revised expressions offer clearer and more grammatically correct statements.