Article from BBC
When we talk about mixed conditionals, we are referring to conditional sentences that combine two different types of conditional patterns. These combinations are not all that frequent, but the most common combination is when we have a type 3 conditional in the if-clause (if + past perfect) followed by a type 2 conditional (would + infinitive) in the main clause.
mixed third / second conditional
With this combination we are contrasting an imagined or real event in the past with the present result of that. Consider these examples:
If he'd taken the medication as prescribed, he wouldn't still be lying sick in bed.
If she'd taken reasonable precautions, she wouldn't be pregnant now.
If he hadn't run after the car thief and suffered a heart attack, he'd probably be alive today.
Note that we can also convey the same idea of past event and present result by using type 3 conditional (if + past perfect, would've + past participle) in both clauses
If he'd taken the medication as the doctor ordered, he would've recovered by now.
If she'd taken reasonable precautions, she wouldn't have got herself pregnant.
If he hadn't run after the car thief and suffered a heart attack, he wouldn't have collapsed and died.
Note that we use this type of conditional when we regret past action or inaction.
mixed second / third conditional
The other possibility, though I think this is less common, is when we have a type 2 conditional in the if-clause (if + past simple) followed by a type 3 conditional (would've + past participle) in the main clause.
With this combination, we are describing ongoing circumstances in relation to a previous past event. Consider these examples:
If you weren't such a poor dancer, you would've got a job in the chorus line in that musical.
If you weren't so blind to his faults, you would've realised that he was out to swindle you.
He's old enough to come home by himself, but can you just see him across the busy road?
if + present simple, will + infinitive:
If I wait for Jane, I'll be late for school
This is the pattern that we most frequently associate with the first conditional, referring to future possibility or probability. But note that other patterns are also possible: we can have a modal verb, typically can, may or should, in the if-clause or main clause, as well as going to future or present continuous future. Present perfect is also possible in the if-clause. Consider these examples:
If you can't understand the instructions, you'll never be able to assemble the wardrobe.
If I give you ten pounds, could you get me some wine at the supermarket?
If you've finished the work I gave you, you may go home now.
If the weather's good on Sunday, we're going to have a picnic in Hyde Park.
If you're going to write him a cheque, make sure there's enough money in your account to cover it.
If you're coming clubbing with us tonight, you'd better get ready now.
In this final example, note that had better is not a past tense. It refers to the immediate future and we use it to give strong advice as the preferred alternative to must, ought to or should.
if you should… / if you happen to…
Note that we use should in the if-clause in the first conditional if we want to suggest that something is very unlikely. We can use happen to in a similar way or even combine them:
If you should / happen to change your mind about coming to the beach tomorrow, give me a ring.
I don't expect him to, but if he should happen to show up, whatever you do, don't let him in!