The following are what you should not do for safety reasons.

1. You must not run in the hallway.

2. You must not throw away trash into the river.

3. You must not fight with your classmates.

4. You must not carry matches to the mountains.

5. You must not make a fire in the mountains.

5-1. You must not make a fire in the mountain.

5-2. You must not make a fire in a mountain.

5-3. You must not make a fire in mountains.

(Which ones are correct from 5 to 5-4?)

6. You must not play with a knife.

7. You must not cross the street at the red light.

8. You must not go into the sea without warm-up exercise.

9. You must not throw a stone at your friend.

(Are the expressions all grammatical? If there are any errors, correct them,please.)

2. You must not throw trash in the river.

5-2 is correct.
7. You must not cross the street when the traffic light is red.
8. The sentence is OK, but I suggest: You must not go into the sea without warming up.

I think 5 sounds better than 5-1, 5-2, or 5-3.

Bob Pursley's corrections for 2, 7, and 8 are good ones!!

=)

Out of the options given, the correct phrasing for each item is as follows:

5-1. You must not make a fire in the mountain.

All the other options are incorrect.

Regarding the correctness of the expressions:

1. You must not run in the hallway. - This expression is grammatically correct.
2. You must not throw away trash into the river. - This expression is grammatically correct.
3. You must not fight with your classmates. - This expression is grammatically correct.
4. You must not carry matches to the mountains. - This expression is grammatically correct.
5. You must not make a fire in the mountain. - This expression is grammatically correct.
5-2. You must not make a fire in a mountain. - This expression is grammatically incorrect.
5-3. You must not make a fire in mountains. - This expression is grammatically incorrect.
6. You must not play with a knife. - This expression is grammatically correct.
7. You must not cross the street at the red light. - This expression is grammatically correct.
8. You must not go into the sea without warm-up exercise. - This expression is grammatically correct.
9. You must not throw a stone at your friend. - This expression is grammatically correct.

In conclusion, all the expressions are grammatically correct except for 5-2 and 5-3, which should be corrected to "You must not make a fire in the mountain."