A watch not worth a penny was being sold there.
In this sentence, do I have to, must and should put "that is" like this?
A watch that is not worth a penny was being sold there.
A watch not worth a penny was being sold there.
In this sentence, do I have to, must and should put "that is" like this?
A watch that is not worth a penny was being sold there.
A watch not worth a penny was being sold there.
In this sentence, do I have to, must and should put "that is" like this?
A watch that is not worth a penny was being sold there.
No, you should not. The reason is that you cannot assume the present tense. The watch wasn't worth a penny at the time but you don't know how much it is worth now.
Therefore you can either leave the sentence unchanged, or you can add 'was', as follows:
A watch that was not worth a penny was being sold there.
I prefer the original, shorter version.
You may, but it is perfectly idiomatic without it. It is a small clause.