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 answers

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!!

=)