Is this sentence correct?
It would be great if he is coming.
(At the time of talking)
Since you are using an “if” clause,
you need to use a past tense there with “would”,
“It would be great if he came.”
You write this when there is little possibility of his coming.
But if there is a real possibility of his coming,
“It will be great if he comes.”
If you are certain that he is coming,
“It’s great that he’s coming!”
“Would”, however, is not just used as the past tense of “will”. If you look up any advanced learner’s dictionary, you’ll find many different uses of “would”.