Left and right subscript / superscript

There are many tools for scripts:

• For general math scripts before something, use \prescript provided by mathtools package, for better alignment:

  $ \prescript{14}{2}{\mathbf{C}} $

• For large operators, use \sideset from amsmath:

  $ \sideset{_a^b}{'}\sum A_n $

• For chemical equations, use mhchem package:

  \ce{^{227}_{90}Th+}

• For tensors, use tensor package:

  $ M\indices{^a_b^{cd}_e} $
$ \tensor[^a_b^c_d]{M}{^a_b^c_d} $

Please complete this list as you can.

Are you doing this in math mode?

${}_{t}p_{x}$

should work.

The tensor package will do exactly what you require, and more. For your particular case, you need only type

\tensor[_t]{p}{_x}

Regards