I have trouble understanding the structure of this sentence

Question

English is my second language, and I'm having trouble understanding the structure of the highlighted sentence down below.

"In his description of how he became a master riverboat pilot in Life on the Mississippi, Mark Twain gave us a vision of learning that incorporates basic skills and the capacity to go beyond them. Twain learns how to navigate the river at young age, learning every shoal, snag, and sandbar, which are the basic skills. But no sooner has he memorized their locations and peculiarities than he has to modify or forget them, and learn other spots, for the river never stops changing its course. Twain must simultaneously remember the reality of what exists in the river and imagine how different forces and conditions are likely to change it. He can never know the river completely or certainly because knowledge about the river is always temporary."

I know that the sentence is inverted, but I don't understand the use of 'than'.

Answer 1

Changing the word order of the original,
He has memorized no sooner than he has to modify or forget.

"A is sooner than B" means A occurs at an earlier time than B.

Thus, the meaning is
He must modify or forget as soon as he memorizes.

It's an exaggeration, that means that the information changes almost as soon as it is learned.

Here is a simpler example of the usage:
No sooner did I put my umbrella away than it began to rain.
(It began to rain as soon as I put my umbrella away.)

Answer 2

This sentence uses the construction "No sooner X than Y", which is:

used to show that one thing happens immediately after another thing:

• No sooner had I started mowing the lawn than it started raining.

It's another way of saying "Y as soon as X" (or, as a general statement, "every time Y, X"). The dictionary example above could also be written:

• It started raining as soon as I started mowing the lawn.

So we can also rewrite your sentence about Mark Twain:

He has to modify ... and learn other ... as soon as he has memorized ..., for the river never stops changing its course.

The meaning should be clear now, but a more idiomatic way to write it without the "no sooner ... than" construction is:

Every time he memorizes their locations and peculiarities, he has to modify or forget them and learn other spots, because the river never stops changing its course.