"The weapon were used in the war. It was very useful."
If I want to change it to particiiple clause, which sentence is correct grammatically?
1.The weapon used in the war was very useful.
2.The weapon being used in the war was very useful.
How to determine?
I would rewrite sentence number two this way: The weapon being used in the war has been very helpful. Or you could write or say it this way: The weapon that was used in the war was very helpful.
I was hoping that the OP would think about why sentence 2. was not good, and maybe come back with further questions.
The question you should ask is:
Did the weapon achieve its purpose?