i wanted to know if it's okay to use future perfect (using WOULD not WILL) with Past Simple. i wanted to say if i knew he was ill at a point in the past , i will visit him at this point in the past.
It's understandable, but not quite the normal verb tense you'd want to use.
Instead, it would more commonly be:
If I had known you were ill, I would have visited you.
That describes a past hypothetical.
For a present hypothetical:
If I knew you were ill, I would visit you.
And for the future:
If I know you are ill, I will visit you.