how nice

“Just think how nice it would be to see them again.”
Is it correct sentence?
Which one is subject, verb and object for this sentence?
Please elucidate.

Yes, it’s correct.

I think the subject is debatable. If you consider it as an instruction (imperative) then the subject is the person being addressed, and the main verb is “think”. However, if you consider “just think” to be an elaboration of an exclamatory “how…”, with no literal meaning intended, then I suppose the subject would be the dummy subject “it”.