Complex sentence: It is still unknown whether the 1,2-hydrogen shift or the 1,2-carbon shift occurs first, however as 1,2-hydrogen shifts in carbenes usually occur more readily than 1,2-carbon shifts, it has been considered to be the first step of the reaction.
Split into simpler sentences: It is still unknown whether the 1,2-hydrogen shift or the 1,2-carbon shift occurs first. However as 1,2-hydrogen shifts in carbenes usually occur more readily than 1,2-carbon shifts, it has been considered to be the first step of the reaction.