How do I write subscript

There are a number of ways

T_n    means T subscript n

T_n   I wrote this by writing  Tn and then high lighting the n and and then choosing subscript from the drop down format menu.  After you do this though it will stay in subscript format until you substript again and that will cancel it out.  It is kind of weird.

I would do it using the LaTex Formula button.  I would put T_n in the input box and then press ok.

This is the output.  

$$T_n$$