Is the following sentence correct?
We refine it when we can revisit a vertex.
The “it” refers to nothing (it seems to be the dummy pronoun), but the sentence seems correct to me. I think “it” is needed, because “refine” is a transitive verb. What I want to say in this sentence is that we refine the circumstances when we can revisit a vertex.
Is there some grammar rule that would back my claim that the sentence is correct?
“it” refers to nothing
No! This is not true. The “it” probably refers to a term mentioned in the previous sentence. If this was indeed a standalone sentence, then “it” shouldn’t be used.