I'll be surprised unless he has an accident.

[nkaper]

the exercuse asks to find and correct mistakes if there are any:
The way he drives, I'll be surprised unless he has an accident soon.

the correct sentence is "The way he drives, I'll be surprised if he doesn't have an accident soon."
but I don't see why "I'll be surprised unless he has an accident soon." is incorrect. Could somebody explain?

 

[Glenfarclas]

I'm not sure there's any explanation to give you except that we don't say it that way. I don't see anything grammatically wrong with "unless," but it doesn't produce a sentence that an English speaker would use.

 

[nkaper]

I made a mistake in what i wrote as correct sentence. now i edited the first post "the correct sentence is "The way he drives, I'll be surprised if he doesn't have an accident soon."
that is what i wanted to write.

so
The way he drives, I'll be surprised if he doesn't have an accident soon." - correct
The way he drives, I'll be surprised unless he has an accident soon. - incorrect

of course the correct sentence is better and more logical, but the incorrect one has some logic in it too.

 

[Pdstelle]

If using Unless:

He will have an accident, unless he
drives more carefully.

" Unless" indicates a necessary condition for something to happen (or not).

Your sentence may mean that
The only way you can be surprised is by him having an accident.

 

[manfy]

The way he drives, I'll be surprised if he doesn't have an accident soon." - correct
The way he drives, I'll be surprised unless he has an accident soon. - incorrect

of course the correct sentence is better and more logical, but the incorrect one has some logic in it too.

Yes, of course! There is some kind of logic to it -- unfortunately, in this case it's a strange and somewhat illogical logic.

Different example:
I'll divorce you if you go out drinking again with your friends tonight.
-> Normal conditional sentence: If you do this, then I will do that.
I'll divorce you unless you stay home [and don't go out drinking again with your friends]
-> Here I see "I'll divorce you" as an assertion. The speaker has made up his/her mind. 'unless' introduces the only thing that can prevent the main statement "I'll divorce you" from happening.

Compare this to your sentence:
"I'll be surprised unless he has an accident soon" -> The assertion "I'll be surprised" in this context is strange in itself, but setting a condition to prevent this (unspecific) surprise with the word "unless" AND a negative condition (= accident, maybe even death) is just morbid and extra strange and in a way illogical.

[cross-posted]

 

[wandle]

"I'll be surprised unless he has an accident soon."

Your sentence may mean that
The only way you can be surprised is by him having an accident.

No. All it can mean is that the only way the speaker will not be surprised is by his having an accident.

'Unless' is not the right choice here, because it expresses, as Pdstelle says, a necessary condition, a requirement that must be met for something else to be true. It is simply not appropriate for this context.

No one would want to say 'I require him to have an accident in order to save me from being surprised'.

 

[Pdstelle]

I'm sorry. I meant "not having an accident".
Thank you