Subscript in Word
To subscript Word, type the word in the usual way. For example H2SO4. Highlight the character or characters you want to change, and press CTRL, Shift, and 0. It is the zero on the standard keyboard, not the numeric one. It will look like this:
Access from menus
Depending on your version of Word and your Windows or keyboard language, the key combinations may not work. In that case the most practical is to use the menus.
Write the word that contains the subscript or superscript in the standard way, and highlight the letter you want to change. In old versions of Word go to the Format menu , and go to Font. In modern ones, look for the tool ribbon, go to the Home, Format and Font tab.
Inside Font are the Subscript and Superscript icons. Press one or the other as you need: