Hi, zuotengdazuo, and welcome to the G.E,
Context 1: Suppose today is May 29, 2017
1. If John F. Kennedy was alive, he would have turned 100 today.
2. If John F. Kennedy had been alive, he would have turned 100 today.
Context 2: Today is Aug 14,2019
3. If John F. Kennedy was alive, he would have turned 103 next year.
4. If John F. Kennedy had been alive, he would have turned 103 next year.
Hi. Are all the above four sentences correct in the context? I make them up. Thank you.
IMHO, sentence one is fine. It is what is called 'mixed conditionals'. John F. Kennedy isn't alive today, which means it is a counterfactual situation in the present, so you use the past simple (subjunctive mood). I see that the main clause here should be 'would turn', but 'would have turned' could also work if you mean at an earlier time today. You can write it like this:
If John F.Kennedy was / were live, he would turn / would have turned 100 today.
Sentence two doesn't work because of using the past perfect after 'if', which means you are thinking of a counterfactual situation in the past (subjunctive mood), while here you are thinking of the present. I mean that sentence could work if your supposition is related to the past. Today is 14th August, 2019. Then you can say:
- If John F. Kennedy had been alive in 2017, he would have turned .... today.
Sentences 3 doesn't work because of using 'would have turned ..' while you are thinking of the future.
Sentence 4 is meaningless.
BTW, your question reminded me of one Michael Jackson's songs called 'They don't care about us'. In one part of it, he says:
But if Roosevelt was living, he wouldn't let this be.