soon
Pronunciation: /suːn/
adverb

Phrases
-
no sooner —— than
- used to convey that the second event mentioned happens immediately after the first: she had no sooner spoken than the telephone rang
-
sooner or later
- at some future time; eventually: you’ll have to tell him sooner or later
-
sooner rather than later
- before much time has gone by: I would be grateful if you would come to your senses sooner rather than later

Origin:
Old English sōna 'immediately', of West Germanic origin

In standard English the phrase no sooner is followed by than, as in we had no sooner arrived than we had to leave. This is because sooner is a comparative, and comparatives are followed by than ( earlier than; better than, and so on). It is incorrect to follow no sooner with when rather than than, as in we had no sooner arrived when we had to leave.