INSERT SUBSCRIPT FROM GOOGLE DOCS TOOLS
To do this go to Toolbar and select Insert from the menu then Special Character from the appeared menu.
SUBSCRIPT IN GOOGLE DOCS
Write the base and subscripts element together and select the element that will be the subscript then go to the Format tab and select Text then click Subscript option.
SUBSCRIPT USING KEY COMBINATION
Follow the same process which described above and select the element that will be the subscript then press the CTRL with + key
To remove the subscript, you just have to select the subscript element and press the CTRL with + key.
Note: To insert Superscript in google docs we use the same process.