How to write pow math?

Embrace the {w^{\mathrm{anterior}}} rather than the {i^{w}}. Also, \mathrm for math-sized text (by "text", I mean phonetic language, not a collection of math symbols).

\documentclass{article}
\begin{document}
$i^{w^{\mathrm{anterior}}}$
\end{document}

If I understood you correctly, you want the superscripts have normalsize plus an sf shape, you can use \mbox.

\documentclass[12pt,a4paper]{article}
\begin{document}

\newcommand{\mb}[1]{\mbox{\sffamily#1\,}}
$\mb{i}^{\mb{w}^{\mb{anterior}}}$

\end{document}

Another good approach is \operatorname{anterior} from amsmath, especially if you want two of these variables to appear consecutively.

Personally, I would prefer to define \newcommand\anterior{\operatorname{anterior}}, analogously to \sin or \log.