Can you use "when’re"?
Theoretically, yes you can; although, as abbreviations go, it isn’t terribly efficient. The missing letter marked by the apostrophe still has to be voiced when saying the word aloud (unlike more established abbreviations such as we’ll and there’ll) and in writing, it saves the writer only one written character.
The Oxford English Corpus does not give very many examples of the ‘when’re’ construction, further backing the suggestion that it isn’t very useful.
To learn more about contractions and other abbreviations, visit this page.
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