The following question is from a placement test and the answer is "When".
Would you please help me why "As" cannot be used? Thanks and regards.
_____ he was walking down the street, he ran into Jane.
We often get confused students here with similar questions about tests. (Not from Turkey though). The most likely possibility is that the question was written by a non-native speaker who doesn't realise that "As" is also correct (and in fact, better).
Other possibilities are typo, transcription error.
Now that you know "As" is correct, you could ask Sabanci University why their answer is wrong.
They are stipulating 'when' because the second 'action' - he ran into Jane - does not happen during the same time period as the first : 'he was walking'. It is either the end of the time period (he stopped walking to talk to Jane) or an interruption to it. They are not coincident =occurring together in time.
"As she grew older, she kept more and more to herself."
If "when" is correct, both i) walking down the street and ii) running into Jane, happen at the same time. That is, they are coincident. When he ran into Jane, he was walking down the street. If they occur at the same time, they necessarily occur during the same time period.
Would you object to "While he was walking down the street, he ran into Jane"? Are you claiming that "As" is wrong?
walking..........................|stop/Jane|................maybe walking again
(happening before)^^^^^^action stops^^^^^^^action may continue
(............As.... grew.... older.............)
(........kept.... to.... herself.... more......) coincident occurrences
Dear Teachers, thank you two so much for the efforts to clarify the point. I'll notify the authorized person at that institution of that question, referring to the explanations made here.
...and please ask them their 'authoritative' (as far as those taking the test are concerned!) reason.