Only then can we know

I expect there is a deeper grammatical reason, having to do with English at its roots, but the quick, superficial answer I submit to you is that this is a poetic construction. It's a rhetorical device and elevates the sentence. Sort of like an exhortation you could imagine, like, "This must we do, to achieve our ends."

As xylo said it is called inversion and is rooted in Germanic languages. In today’s English it is used as an emphasis when one wants to be formal. In Danish for instance it is very common and you’d hear it a zillion times a day

Interesting. Although I speak English as a second language, I have always taken pride in the fact that I I speak it quite well, to my own opinion. The strange thuing is that I construct these sentences in the same way in English, but had never thought of the reason. Now, thinking about it, I guess it must be some underlying V2 rule in English. In Dutch we alsay invert the verb and the object of an independent clause, when the verb isn't in first place, because of the v-2 rule in our SVO word order.
Since English is also a Germanic language, it isn't very surprising that, although the word order in English is much simpler than in Dutch, it shares some of the same characteristics.

There are several of these older constructions:

Never have I seen such a beautiful sight.
Rarely do we ever get to do something important.
Only if he does this will he have the results.
Only when.....
Seldom do I work more than two hours.

There is also:

Not only does he speak French, but he also speaks Vietnamese.

Seems to be a carry over from more archaic structures and is certainly an emphatic or poetic style.

Reminds one of Yoda-speak:

Tired I am.

English structure used to have objects first...and word order was quite different several hundred years ago.

I am no expert, but I do have a question (or two) that I hope could be answered. With this inverse, are such sentences considered to be grammatically correct? And if so, would you consider them to be "passive"? Thanks...

Yes they are grammatical. And no, they're not passive.

People are saying that inversion is for emphasis or garnish, but I was just thinking about Jim's example sentence, "Not only does he speak French, but he also speaks Vietnamese." The inversion, here, looks necessary to me. "Not only he does speak French, but he also speaks Vietnamese" sounds like a syntax error to me. Maybe inversion is not just poetic? Maybe we need it?

One thing common to all* these sentences is that the inversion of SV to VS allows the sentence to begin with the adverb(ial)—which in all of these examples is temporal: (not) only, rarely, never. Furthermore, the only verbs inverted are auxiliary (do, have, can), not main verbs.

Some sentences can be re-inverted to standard SVO order, but this moves the adverb(ial) after the verb, and makes many of the sentences sound awkward.

Not only does he speak French...
He does not only speak French...

Never have I seen such a...
I have never seen such a...

but

Rarely do we ever get to do...
We do rarely ever get to do...

No sooner had he got a bath than...
He had got a bath no sooner than...

Placing the adverb at the beginning of the sentence greatly changes the emphasis, but require the inversion of subject and auxiliary verb.

Personally, I reserve SV inversion for questions, and avoid constructions like this wherever possible.

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*not all of the sentences in the comments follow this construction:

"Tired I am." and "Such a brilliant pianist was he..." are inversions of subject and complement, not subject and verb. However, "Rarely did he find such a brilliant pianist that..." follows this construction.

"Only if he does this..." begins with a temporal adverb, but does not invert SV.

It's the subjunctive mood, expressing a condition of unreality. We teach it in Spanish as well.

I've a quiestion for you. I'm studying english and i've homework. There is an exercise that i can't make

This is the sentence:
He received a letter a few days ago and did not learn till then that he had won a lottery

How do i change the sentence?

Please help me...

Less, What do you mean by "change the sentence"? You could use a different font, or use pretty colors, different ones for every letter. You could translate it to another language or you could write it backwards by looking in a mirror while you write it. You chould change all the vowels to consonants and vice versa or you could just change "letter" to "spanking" and change "won a lottery" to "missed his mistress".

Some of these sound better than others

He didn't learn until a few days ago, when he received a letter, that he had won the lottery.

After receivng a letter a few days ago, he learned that he had won the lottery.

He learned that he had won the lottery a few days ago when he received a letter.

He learned that he had won the lottery when, just a few days ago, he received a letter.

When he received a letter a few days ago, he learned that he had won the lottery.

Just my $.02

Gee, Austin, what would make any of those sentences better than the original? Which brings me back to my original question to Less, change the sentence in what way? for what reason? Less, please clarify! What is it you are trying to do? Oh, and if there were a version I would prefer, it would be "He did not learn that he had won the lottery until he received a letter a few days ago."

touche... I like yours best.

I didn't say mine were better than the original, I was just offering some possible changes. Who knows...maybe Less already turned in the assignment!

I need your help for this question.
* the position of adverb
* how to form an adverb from an adj, noun, verb
* the function of adverb

Take a look at the original, and Jim’s examples. What are the verbs here? Can, have, do. What do these all have in common? They are helping verbs. The main verb remains after the subject. For instance, in the original sentence, the main verb is “achieve”, not “can, and it is “we achieve”, not “achieve we”. “Only then” modifies “can”, and is thus put next to “can”. To do this, “we” must be put after “can”. Only helping verbs can be inverted in this way. Take, for example, the sentence “He speaks French”. If we want to add “not only” to that, what are our options? We could say “He speaks not only French”. But what if we want to have “not only” modify the verb rather than “French”? We can’t say “Not only he speaks French”, because “not only” is supposed to be modify the verb, not the subject. But we can’t say “Not only speaks he French”, because “speak” isn’t a helping verb. So we add the “dummy” verb “do”, and invert it. “Not only does he speak French”.

Here’s another example. Suppose we want to say that we DID receive funding, and we achieved our goals only after receiving funding. Would we say “Only then we achieved our goals”? No, because “only then” modified the verb, so it should be next to the verb. What about “Only then achieved we our goals”? No, because “achieved” is not a helping verb. So we add a helping verb: “Only then did we achieve our goals”.

The use of an adverbial in front position REQUIRES inversion, this occurs not only for the sake of emphasis (altough the resulting sentence is much mor emphatic), but also to thematize an object, or in your case the adjunct of time. This is important as regards organization of information in discourse (or information management as grammarians call it.)