pronoun
1
BAD : When hearing that the child had been found, she burst into tears.
GOOD : On hearing that the child had been found, she burst into tears.
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To show that two things happen at the same time or that one thing happens immediately after the other, use on/upon doing sth (NOT when ): 'On examining the suitcase, he noticed that the locks had been tampered with.'
2
BAD : When I'll return home, I'll look for a better job.
GOOD : When I return home, I'll look for a better job.
3
BAD : I don't know when is she coming.
GOOD : I don't know when she is coming.
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See note at WHAT 3 (↑ what )