What's the difference between "happen" and "occur"?

[Hann]

English Language Tests, Intermediate level

ESL/EFL Test #87 "Simple Present (4)", question 8

What was ......... to him? He saw Sally standing on the platform getting into the train and the train leaving the station.

(a) happening
(b) occurring
(c) falling
(d) coming

English Language Tests, Intermediate level

ESL/EFL Test #87 "Simple Present (4)", answer 8

What was happening to him? He saw Sally standing on the platform getting into the train and the train leaving the station.

Correct answer: (a) happening
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What's the difference between "happen" and "occur"?

[Sachithra]

What's the difference between "happen" and "occur"?

[Beeesneees]

You will find the answer in Alan's reply here: Occur vs. happen
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Cheers m' dears!

[Kati Svaby]

Examples with to occur.
-When exactly the incident occur?
-Something unexpected occurred.
-Sugar occurs naturally in fruit.

phr.verb: occur to sb =(of an idea or a thought) to come into your mind.
The idea occurred to me in a dream.