We have two different kind of substcripts: the literal subscript which you get with the keystroke CTRL-minussign and which is just part of the name. You can't use varaibles as index or do calculations like you tried. You cannot access matrix elements with the literal subscrpit.
The secon subscript is what you probably need, its the vector or matrix subscript you get with CTRL-[. This subscript is used to asign or access the elements of matrices and of course variables and calculations like i-1 are allowed.