"If David didn't give up smoking, Liza wouldn't marry him." I only can see that sentence in the 3rd conditional (unreal past) - "If David hadn't given up smoking, Liza wouldn't have married him". But I don't understand its meaning in the 2nd conditional. Is it just me, or this sentence doesn't work in the 2nd conditional?
Let's look at this in the present tense:
If David does not give up smoking, Liza does not marry him.
To me, this sounds a bit informal, but not necessarily incorrect. I would say:
If David does not give up smoking, Liza will not marry him.
To illustrate the usage of the former, imagine a question and answer scenario, like this:
And what happens if David doesn't give up smoking?
Then Liza doesn't marry him. Now quit your worrying!
Again, what will happen and will not marry could be better choices, but dialogue is often imperfect.
With these insights, let's send the verbs back to the past. *time travel noises* Once more we have:
If David didn't give up smoking, Liza wouldn't marry him.
Let's imagine another question and answer scenario:
Why wouldn't Liza marry David?
He didn't quit smoking. Never had much self-control.
I 'reversed' the exchange because we can now distinguish between the cause (David did not stop smoking) and effect (Liza would not marry him). A cause and an effect can form an 'if-then' statement, which is what you are reading.
If [cause], then [effect].
If David didn't give up smoking, [then] Liza wouldn't marry him.
Hope this helps!
The way I see it, the reason that sentence is in the SECOND Conditional is because THE OPPOSITE context happened, making the situation specified in your sentence IMPOSSIBLE to happen anymore.
So, if you say: "If David didn't give up smoking..." <- the fact you mentioned the conditional clause in the simple past means that this is something unlikely to happen, which is what the 2nd Conditional is all about. Therefore what happened is the opposite, or: David gave up on smoking and Liza married him.
Now, to your question: Yes, it also makes sense to work with the 3rd Conditional, but that would imply that David either didn't care about marrying Liza or he could not quit smoking. So currently, he still smokes. So now we use the 3rd Conditional to express that David REGRETS about his decision or misses Liza. That's what the 3rd Conditional is all about, expressing a past regret (or in other words, a regret about a decision made in the past). Therefore: "If David had given up smoking, he would've married Liza".
I hope that helps you out.