I have encountered similar problems in different versions of Word, depending on version number, UI interface language, keyboard layout, and maybe other factors. There are various solutions (not mutually exclusive):
- Use mouse commands to select text and set its font to subscript (or its style to a character style defined to use subscripting).
- Define your own keyboard shortcut for subscripting (either so that it directly applies subscript style or so that it sets the style), e.g. via Word settings, Customize Ribbon (in Word 2013). This might be the most convenient way. You can then select a combination you remember easily and is not needed for any other purpose, e.g. AltS (unless you have some add-in that uses it).
- Use Equation Tools. They are suitable if you are writing mathematical expressions. They can be used, but much more clumsily, even when writing things like COâ‚‚ (you need to change the default italic for letters to upright style).
- Use subscript characters such as U+2082 SUBSCRIPT TWO â‚‚ (which you can enter e.g. by typing U + 2 0 8 2 Alt X (not very convenient, but you can define a shortcut or an autocorrection or switch to a special keyboard layout).
- Define autocorrections for subscripts that you use frequently, e.g. so that _2 is turned to the â‚‚ character.
Subscript characters generally produce better typographic results when they are available. They are designed for such use, whereas the subscripting made by Word just takes a normal character, reduces its size, and lowers its position. Subscript characters exist for a limited set of letters and digits and a few other characters, and they are not included in all fonts, though. Equation Tools usually produce typographically good results, too.