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Assume there is a sentence like:

I don't think he doesn't know why he shouldn't have left her alone.

When it is in written form, at least, I have chance to re-read it and thorougly examine, apply negative+negative = positive, etc.. But when it's in conversation with fluid native speaker, there is no such a chance.

How do you cope with it? Are there any tips? Though I think there aren't any and the best way is to practice more, but hope dies the last.

Thanks.
Comments  
Well, would you have trouble perceiving this sentence in your native language? I suppose, no. That's the solution: learn English until you develop "a hardware support" for it, i.e until language processing is shifted beneath the conscious.

BTW, double negation, while working in Boolean logic, doesn't always apply to predicates! So:

«I don't know why he shouldn't have left her alone» is not equal to
«I know why he should have left her alone»

HTH
Oleg_l I don't think he doesn't know why he shouldn't have left her alone.

Hi,
that's a difficult one. I believe most natives would not understand and say "What the...?"
Without a context. As it is. But put that in a context, and you don't even have to listen to the whole sentence to understand.

A) He shouldn't have left her alone.
B) Yeah, of course, but I believe he doesn't know why he shouldn't have left her alone.
A) Hmm, no way... I don't think he doesn't know why he shouldn't have left her alone.

This last one can be rewritten as:
I don't think blah blah = I don't agree.
You don't need to listen to the rest, they don't agree, that's the point. That's why I don't think most natives would ever say I don't think he doesn't know why he shouldn't have left her alone, but would replace it with I don't think so/that's true/it's actually that way/etc.

I think natives avoid using too many negatives all together. That would be confusing, also because double negatives are sometimes/often really negative in meaning. So if you don't understand nothing, you really don't have a clue. Emotion: smile
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Okay, native speaker here. If I heard that, there's about 95% chance I'd say "You don't think he doesn't know what? Huh?!" It's a horrible sentence.

Unless the prior conversation had gone like this:

A: He shouldn't have left her alone.

B: Yes, but he claims he didn't know about her fear of the dark.

A: You're saying he didn't know he shouldn't have left her alone there?

B: Yeah, that's what he says. That he didn't know why she shouldn't have left her. He didn't know about her fear.

A: Ha. He's a jerk. I don't believe he doesn't know... etc.
Grammar Geek there's about 95% chance I'd say "You don't think he doesn't know what? Huh?!" It's a horrible sentence.
Ha! I was right! Natives wouldn't understand either!
GG, you thought of an example to add a context... as I did. But I also added I think most (probably all) native speakers would change that sentence to avoid repeating so many negatives... and keep it short:

Ha. He's a jerk. I don't believe he doesn't know he shouldn't have left her alone
---> I don't believe he doesn't know that.
---> I don't think so.
---> I don't think that's true.

Do you think that makes sense?
Emotion: smile
Let me add my two cents.

I don't think native speakers generate sentences like that -- certainly not very often.

I find that sentence absurd. I have no idea what it means. I don't even think it's worth the trouble to figure it out and invent a context where it might have some small probability of being comprehensible.

It's not only negatives with the word not, but also verbs that have a negative component, like doubt, deny, avoid, forget, prevent, and so on, that can give the listener trouble. I try not to use too many of them all in one sentence.

I doubt I would deny having forgotten to avoid neglecting it,

for example, is a nightmare. I don't know if it even makes sense.

CJ
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Kooyeen, yes, in that silly scenario I wrote (as an example of just how detailed it would have to be to make sense of it), we would not repeat it all. You'd say something like "I doubt he really didn't know," just as you suggested.

Ant, seriously - you'd be able to follow that in your native language?
GG: «Ant, seriously - you'd be able to follow that in your native language?»

Yes. As well as in written (not sure about spoken) English. These negations are "nested", but there are worse situations like:

I don't know that he doesn't know that I don't know that he doesn't know...

Those are really terrible recursive structures that I am not able to unravel...

CJ's last example is also difficult.