Can you also use 'mens' in this context?
hmm, i dont think so, it means 'while' ^^ there is a little difference between them what's sometimes important
How do I know when to use når and when to use mens?
In this sentence, when can easily be replaced by while. Without any context, both når and mens should be acceptable. See the other Duo Lingo sentence: the children play when the woman works, where when is translated as mens
Don't when/while have the same meaning?
Not precisely. He jumps when the gun goes off, but he breathes while he's alive. Either word works in "the dog howls when/while the lady sings".