Why was I told 在什么情况下……吗？ is wrong?
“吗” is used in &quot;Yes-no questions&quot;, which are question that can be answered by &quot;yes&quot; or &quot;no&quot;.
With &quot;Under what circumstances&quot;/&quot;在什么情况下&quot;, the question cannot be answered by &quot;yes&quot; nor &quot;no&quot;. So &quot;吗&quot; should not be used.