*For the current problem: Click after the subscript text, then press delete until you have only Plain Text at the insertion point. Proceed as below.
*In regular use: Type the subscript text as part of the Plain Text, and continue beyond it for a few characters. Then select the text that is to be set as subscript, and change its format, using the Text menu or the usual keyboard command. This wil change only the selected text, and will leave the followg text as Plain, a format that should hold as you continue typing from the end.
Regards,
Barry