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Can a sentence begin with upon? Or should it be changed to on?

For example:

  • Upon a decrease in temperature... [Is this grammatical?]
  • On a decrease in temperature... [What about this?]
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Sure it's fine. A little old-fashioned though. – Mitch Aug 14 '12 at 12:53
I'm not convinced OP's example is really "acceptable" today. As Mitch says, all such usages of upon are somewhat "old-fashioned". But even allowing for that, I'm not comfortable with using upon in contexts when it's synonymous with whenever (as opposed to on the specific one-time occasion). – FumbleFingers Aug 14 '12 at 13:09
From the American Heritage Dictionary: In their uses to indicate spatial relations, on and upon are often interchangeable: It was resting on (or upon) two supports. We saw a finch light on (or upon) a bough. To indicate a relation between two things, however, instead of between an action and an end point, upon cannot always be used: Hand me the book on (not upon) the table. It was the only town on (not upon) the main line. Similarly, upon cannot always be used in place of on when the relation is not spatial: – JLG Aug 14 '12 at 13:55
They are equivalent, and both are well-formed. As a matter of style, however, both are stiffly formal and would almost never be heard in conversation. As FumbleFingers says, these constructions should not be used to describe or prescribe routine actions. "Upon a decrease in temperature, ring the alarm" would feel right, but not "Upon a decrease in temperature proceed to step 2" would not. The most natural use is in historical narrative: "Upon hearing of Armstrong's incursion, Scrope called up his riders." – StoneyB Aug 14 '12 at 14:04
@XavierVidalHernández's comment prompts me to correct myself: these constructions are reasonably natural (if somewhat formal) in present as well as historical narrative, and will accommodate repeated action in this context: "On returning from work he usually [visits/would visit] his mother." – StoneyB Aug 14 '12 at 18:22

There are certain use cases for both, which is why we have both words.

Upon further review I decided to go to the park.

In this example you cannot simply replace upon with on and maintain the same flow.

On further review I decided to go to the park.

So why are they different? Upon has a more literary effect to it and sounds pretty formal in general. As FumbleFingers noted upon can indicate a one-time occasion such as

Upon seeing the bear, I rode my horse into the sunset.

whereas "on" is used less frequently to express one-time occasions.

On seeing the bear, I felt this sentence sounded strange.

Judging from the comments I would say it is a stylistic choice, rather than a grammatical choice.

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I think it is stylistic but I have to say I prefer 'Upon...' Just saying 'On...' sounds incomplete or lazy to me. Perhaps I am just old fashioned. – Barry Brown Aug 15 '12 at 7:02

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