They need a consistent framework to understand what is happening - "variables are like little boxes where you can put a number or some letters" and "The equal sign lets you change what is in the box."

x+=1 just complicates the situation; use x=x+1 to avoid adding more special rules for now.

This stuff is teachable, but it doesn't help that the framework is very different than the similar-looking expressions from math class, where "x=1" followed by "x=2" is not allowed.