Simply.
I start typing in the cell with Text style. I press the buttons:
0, =, x, Ctrl+Shift+-, 1, RightArrow, <, x, Ctrl+Shift+-, 2, RightArrow, <, ., ., ., <, x, Ctrl+Shift+-, N, RightArrow, = , 1
When I type the text next to the formula, it (formula) sometimes triggers to the expression with Subscript[].
Mathematica 10.4, Windows 8.1
To my mind, inline formulae in Text cells should be typed in a specified mode, not just by typing letters, sub/superscript etc.