Translation:He was seen while selling my phone.
To mark "telephone" wrong is laughable. I don't see the objection to "when" or "while" but a native English speaker would probably use neither. "Seen selling my telephone" says it all.
Your knowledge of English is credible...your knowledge of how this app works is sadly lacking... The problem's nothing to do with vocab or grammar...its simply an oversight.
The app tends to force one answer while rejecting other correct and often better answers. That is a flaw not an oversight.
I agree but is it a "dài thun" , or how Vietnamese refer to their sense of time like stretchy, deal that it takes three or more years to correct an oversight?
BTW, I've been living in Vietnam a couple of months and a couple of months more. In HCMC and will be in Song Ve in a few days for about a month. See ya around.
I did not say "while" was incorrect. I said Duo forces one answer while rejecting other correct answers. Sometimes Duo is even wrong. Often the rejected answers are more natural than Duo's. This is something that could be corrected but usually is not.
I moved to Vietnam so I have a lot of opportunities to get clarification of two year old problems here. One of the people I live with is a university professor of engineering. I got him hooked on English here.
Anyhow, best I could interpret our conversation on the phone vs telephone issue, he agrees with Duolingo on using phone and rejecting telephone. He says phone is thought of as just the device you use and telephone is the whole system of cables or towers and whatnot.