"He cried because his mother had forgotten who he was."

Translation: Han gråt fordi moren hans hadde glemt hvem han var.

[Snommelp]

Not moren sin? Or is that a mistake? I don't want to report it, because I'm not so certain of myself right now...

[fveldig]

"fordi moren hans hadde glemt (...)" is a dependent clause, where 'mor' is the subject. 'sin' would refer to the subject, but because 'han' is not the subject of that clause, you'll have to use 'hans'.

[askepka]

Why is for instead of fordi wrong?

[Bunchybunc]

Could we use 'på grunn av' instead of 'fordi', here? Would somebody please remind me when we should use one over the other? I can't find the answer.