Please look at the following sentence.
(1) Who is he?
When this sentence is embedded in another phrase "we can not tell", it becomes,
(2) We can not tell who he is.
(3) We can not tell who is he.
Am I correct up until here?
Then please look at the following sentence that I found in The Boxcar Children series
(4) In those days, we could not tell who was a friend and who was an enemy.
We don't have to change this to
(5) We could not tell who a friend was and who an enemy was?
I feel (5) is somewhat unnatural, but I don't know why and can't explain it to the students very clearly.
I'd appreciate your help.