NNC.

Э-э...зря вы это сделали, мы, вероятно, будем то предложение менять. По крайней мере, "искал" на "ищу".

Может, просто поменяем всё по-быстрому? Netka уже больше года отсутствует, и @change к её предложениям, вижу, не первый раз, но она ни разу не откликнулась - видимо, забыла про Татоэбу. Так что просто взять и поменять время глаголу, а потом пусть Эльдад то же сделает в переводах, пока к ним не появились свои переводы.

For the time being, I can unlink the French sentence from the Russian one. I hope the French can still remain by its own right. Should I unlink it?

Would it not fit if the tense of the verb were changed in Russian? (I know the tenses would be different, but since the way you understood the Russian sentence seems to go with the latter verb being in Present, then they could still be meaningful pairs. We aren't sticking to letter-to-letter translations, are we?)

OK. I believe the tense can be changed, both in the Russian and in the other sentences, linked to it.

However, the principle question remains: why can't we use two tenses in the same sentence? Is the "concordance du temps" so sacred a principle?

It's hardly about sequence of tenses, I think... It just doesn't seem to make sense to say that you can't (now) achieve the result of an action you did in the past. If it failed in the past, then your inability would also be in the past. If it's failing right now or close enough to "now" that the failure is still vivid, then it would be natural to use the Present tense because it feels like the present. Putting the action in the past but its lack of success in the present appears strange, because that means the action is not "relevant to the present" but the fact that it failed is?

Maybe I'm failing to grasp the principle which is so clear to you because in my own native language, we don't have a present perfect. In Hebrew, it's really, literally, "I cannot find the page that I looked for/was looking for" (we also don't have a progressive tense).

So, in Hebrew, you won't use the Present in the clause here?

Doch. :)
But as I could use the present in a parallel Hebrew sentence (I can't find the page I'm looking for), I could use the past as well, beside the first clause, which is in the present tense.