upon

A higher-register or more formal alternative to on in most, though not all, prepositional uses.

How to use

The preposition 'upon' is often employed in formal or written English to convey a sense of immediacy or proximity, and can be used interchangeably with 'on' in many contexts, such as 'upon arrival' or 'upon completion'. However, learners should note that 'upon' is not always interchangeable with 'on', particularly in phrasal verbs where 'on' may be used but 'upon' would sound unnatural. A common mistake is to use 'upon' in informal spoken contexts, where 'on' would be more suitable.