I am a long time Maple user, and like it a lot. It's very helpful in my research.
In the new 2-D Math input mode (Maple 10 or so, circa 2005+), typing an underscore "_" puts one in subscript mode. If you really want to use an underscore in the name of a variable, precede it with a backslash "\".
For example, typing t_0 will give the subscripted variable t, which in 2-D math mode is displayed as t0. But typing t\_0 (the backslash will dissappear after typing the underscore) will give the variable t_0.