Inversion + past tense

There is a sentence:

No sooner had he sat down than he fell asleep.

I just do not understand, is this an inversion? And if so, I still do not understand the sentence.

Solution 1:

The negative no sooner is placed at the front of the clause for emphasis. This construction requires inversion, in which the auxiliary verb, in this case had, is placed before the subject. The unmarked version would be He had no sooner sat down than he fell asleep, and it means that he fell asleep immediately after sitting down.

Examples of other negative words that can be used in this way are at no time, not only, not once, hardly and seldom.

Solution 2:

No sooner than is another way of saying as soon as. I'm not a fan of this construction myself, but it is perfectly correct and fairly common.

I suspect that the author simply felt that

As soon as he (had) sat down, he fell asleep.

didn't sound sophisticated enough.

Edit: Note that if you write this sentence in the "as soon as" order, you have a choice of tenses: simple past tense ("As soon as he sat down"), or past perfect ("As soon as he had sat down"). The "no sooner than" construction requires the use of the past perfect - although it is grammatical to say "No sooner did he sit down than he fell asleep", it sounds incredibly odd.