@NNC intensively.
"It was on that day that I (had?) first wondered if I had made a wrong turn [on the road? path? something else? of life]."?

That day was perhaps the first time I'd thought I had not made a wrong turn on life's road. (?)

O'Sea, look at my attempt (I can read the original, you know. :>), how do you find it?
(If "that day was perhaps the first time" is good, then it fits too; what about the second part? See, the author of this sentence must've been in a hurry and misread a grammatical negative for a real one.)

How about this, perhaps?

That day was perhaps the first time I'd wondered if I hadn't made a wrong turn on the road of life.

English can be funny. The fact that I'm thinking or wondering if I have NOT done something means approximately the same thing as thinking or wondering if I HAVE done something. Perhaps it's the same in Russian as well, Ooneykcall?

That's fancy, and somehow it had escaped my knowledge prior to that point... Why, yes, we're using a negative here in a question to reflect a positive, though there's a difference between using a positive and using a negative: in this case, one would've used a positive to wonder whether they have done something they hope they have, and a negative for something they hope they haven't, as is the case here.
Your suggestion is good and I'm looking forward for this sentence to be changed according to it. :>

Does English feature any differences between a positively-framed and a negatively-framed if/whether-clause?

Sorry, Ooneykcall; I'm a keen editor with a red pen and an itchy finger, ready to correct other people's grammatical faux pas, but some questions are simply above my pay grade, as they say. I personally see no distinction between saying:

I wonder whether I should go to the ball game.
and
I wonder whether I shouldn't go to the ball game.

To me, there is zero difference because implied in each is its unstated opposite as well:

I wonder whether I should go to the ball game (or not).
and
I wonder whether I (should or) shouldn't go to the ball game.

The sentence as it is is quite clumsy.

A native speaker would not say or write "That day, it was probably the first time..." or "when I thought if I didn't make the wrong turn ..."

I agree that the following or something like it would be much better:

>That day was perhaps the first time I'd wondered if I hadn't made a wrong turn on the road of life.

Ok, I've corrected it.
One question, why did all the posters here use "I HAD wondered/thought if I had made a wrong turn"? As far as I know, "had" indicates that the action happened before something. It's evident that "made" precedes "wondered" in time, but you write "had wondered" anyway.

(Хоть я и не носитель, но рискну сказать: this being the 'first time' one 'had wondered', it may have happened later again, and then this one 'had wondered' would precede a non-mentioned instance of wondering. o.o)

Thank you, RU, for fixing up the sentence. You're right that the past perfect "had made" precedes the past perfect "had wondered" in time, but English does not have any means of indicating these relative times through a verb form choice once the need for the past perfect has arisen.

As Ooneykcall points out, there would be, after the first time, other instances of wondering — which would have taken place in the simple past, had they been specifically mentioned in the sentence.

If you find all this tough going, the best thing is probably to follow the oft-repeated advice and concentrate on translating INTO your strongest language. Several of your "English" sentences are not standard English, and many are almost impossible for a native English speaker to understand (#3519211, #3519210, #3512524, #3519182, etc.)

This is not meant to be harsh criticism; I'm sure the Tatoeba community appreciates your efforts, but if your strongest or native language is Russian, for example, you should probably focus on contributing sentences that you've actually heard Russians speaking, and also try to translate into good Russian some of the English sentences that have been contributed by other, native, English speakers.