The base of the natural logarithm is entered as exp(1).
ln( exp(1) );
1
And exp(x) will raise that to exponent x.
There are two basic ways to get subscripted names in output. You can index the name in the input like c[1], or type two underscores after the base name eg. c__1.
If you are entering code in typeset 2D Math input mode (the default for new users) then in (Maple 17 or 18) as soon as you type c__ the cursor position will drop down to the subscript position. Use the right-arrow key to move the cursor right and up out of that position.
If you use the indexed name like c[1] then you cannot also assign to name c without clobbering the former. Also, some commands get confused when both are used together.
So, it's better to use the double-underscore way if you don't need the subscript to be mutable. Better to leave c[i] for the case where you need the i to change value. You mentioned subscripted constants, so likely you want the double-underscore.
Hope that helps some.