There are others here that are more versed in language then myself, but I'll give it a try anyway. It will probably be more confusing after I'm done
I think this sentence is mostly just a play on words so that it would sound cool when read (in english) and reflect the title
Break it into its two parts
1) It is only after that we understand what has come before
2) Then we understand nothing
First part:
"What has come before" is just a phrase used to describe the past. So it means the same thing as:
"It is only after that we understand
the past"
This sentence basically states that you can only fully understand the past after it has happened.
That is saying that while you can predict probable outcomes very accurately, you can never be 100% sure what
will happen until
after it has happened. The general idea is that nothing is certain with 100% probability. After all, the probability that you will win the lottery is essentially 0, and yet people do still win.
The second part:
"Then we understand nothing"
This can be taken with the first half to generally say that there are times that something occurs that is unpredictable or unlikely. There was no way to forecast that an unlikely event would take place
before it happened.
You could then re-write the old sentence to say something like this:
"The future cannot be predicted with absolute certainly, it is only upon reflecting on the past that events can truly be predicted (up until the present). But, since you are only reading history rather than predicting it, you are forecasting nothing, and thus you understand nothing."
I doubt this will be much help but I tried
