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Imagine the following situation. You've just heard that John was in a place at a time yesterday. You infer that there is a possibility that John saw the accident you know occurred nearby at about the same time. Then, is it grammatical to say
If John was there, he would have seen the accident.
?
Only if it's a certainty that being there would cause him to see the accident. If being there only maybe means he saw the accident, then you could use: "If John was there, he might have seen the accident."
This is called a mixed conditional.
— If John was there, he would've seen the accident.
(John is not there, so he didn't see the accident.)
This involved the present with the past. Sometimes, the choice of verb forms in mixed conditionals depends on the time reference, then the above sentence should be rephrased as:
— If John had been there, he would've seen the accident.
— Had John been there, he would've seen the accident. (Inversion.)
(John wasn't there, so he didn't see the accident.)
An example where the present clause and the past clause work as a mixed conditional is:
— If lived in London, I would've gone to the match.
(I don't live in London, so I didn't go to the match.)
Note that mixed conditionals also work by using if + past perfect, would + base form:
— If you hadn't eaten the last meal, she wouldn't be hungry.
(He ate it, so she is hungry.)
It all depends of the verb choice and time reference.
