。
Don't put the cart before the horse.
Literal
Main-and-end [object-を] reverse [prohibitive-な].
本末転倒 ('reversing the main and the incidental') is a 四字熟語 meaning 'getting your priorities backwards.' Here split into 本末を転倒する and used with the prohibitive な — a terse, authoritative command. The English equivalent is 'don't put the cart before the horse.'