- No sooner had I spoken than I wished I kept my mouth shut.
How is it explained that "than" is used alone and without any other words and why "than" in the first place?
As a side question, how to use this construction?
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The idiomatic construction no sooner [X] than [Y] means that event [Y] happened immediately after event [X], with no passage of time between them. Both events happened in the past.
No sooner had I arrived at school than it started to rain.
Event [X] is described in the past perfect tense with an inverted word order.
I had arrived at school.
No sooner had I arrived at school...
Event [Y] is described in the simple past tense.
It started to rain.
...than it started to rain.
No sooner had she sat down than there was a knock on the door.
No sooner had we finished one meal than we started planning the next.
No sooner had I said it than I wished I had kept my mouth shut.
"No sooner" does not always have to go at the start of the sentence. Instead, we can insert it between the auxiliary verb had and the past participle. This is an equally correct sentence. The difference is only a matter of style.
I had no sooner arrived at school than it started to rain.