help understanding formal grammar for subtraction example
T = "0" | "1" means that if you want a T string, 0 or 1 will
do.

E = T "-" E | T means that if you want an E string, a T string
followed by - followed by an E string will do, or a lone T
string will do.

Thus, for example, 1 is a T string, and is thus also an E
string. Hence, 1-1 is an E string since it follows the T "-" E
pattern.

Note that the double quote is just a notational device to distinguish
terminal symbols from non-terminal ones.

Can you go on from here?